ping_handle does not handle errors

[kpanic]

Hi, I discovered this behaviour in mariaex. I used in a test repo version 0.8.4

This issue https://github.com/elixir-ecto/postgrex/pull/258/files seems to be similar.
Was mentioned by @fishcakez on IRC

# Start with db down
iex(1)> {:ok, p} = Mariaex.start_link(username: "beatpanic", database: "ecto_simple", password: "supersecret")
{:ok, #PID<0.180.0>}
iex(2)> 
21:16:35.429 [error] Mariaex.Protocol (#PID<0.180.0>) failed to connect: ** (Mariaex.Error) tcp connect: econnrefused
 
21:16:36.499 [error] Mariaex.Protocol (#PID<0.180.0>) failed to connect: ** (Mariaex.Error) tcp connect: econnrefused
 
21:16:39.048 [error] Mariaex.Protocol (#PID<0.180.0>) failed to connect: ** (Mariaex.Error) tcp connect: econnrefused
21:16:42.791 [error] Mariaex.Protocol (#PID<0.180.0>) failed to connect: ** (Mariaex.Error) tcp connect: econnrefused
iex(3)> Mariaex.query(p, "select 1")                                                                       
** (DBConnection.ConnectionError) connection not available because of disconnection
    (db_connection) lib/db_connection.ex:934: DBConnection.checkout/2
    (db_connection) lib/db_connection.ex:750: DBConnection.run/3
    (db_connection) lib/db_connection.ex:1141: DBConnection.run_meter/3
    (db_connection) lib/db_connection.ex:592: DBConnection.prepare_execute/4
    (mariaex) lib/mariaex.ex:419: Mariaex.prepare_execute/4
# I start the db now
iex(3)> Mariaex.query(p, "select 1")
{:ok,
 %Mariaex.Result{
   columns: ["1"],
   connection_id: #PID<0.180.0>,
   last_insert_id: nil,
   num_rows: 1,
   rows: [[1]]
 }}
iex(4)> # I stop it now  
nil
** (EXIT from #PID<0.178.0>) shell process exited with reason: an exception was raised:
    ** (FunctionClauseError) no function clause matching in Mariaex.Protocol.ping_handle/3
        (mariaex) lib/mariaex/protocol.ex:1139: Mariaex.Protocol.ping_handle({:packet, 30, 0, {:error_resp, 255, 1927, "#", "70100", "Connection was killed"}, <<255, 135, 7, 35, 55, 48, 49, 48, 48, 67, 111, 110, 110, 101, 99, 116, 105, 111, 110, 32, 119, 97, 115, 32, 107, 105, 108, 108, 101, 100>>}, :ping, %Mariaex.Protocol{binary_as: :field_type_var_string, buffer: "", cache: #Reference<0.2383522203.2188771331.72263>, catch_eof: false, connection_id: #PID<0.180.0>, cursors: %{}, deprecated_eof: true, json_library: Poison, lru_cache: {100, #Reference<0.2383522203.2188771331.72267>}, opts: [socket_options: [], sock_type: :tcp, cache_size: 100, timeout: 5000, port: 3306, hostname: "localhost", username: "beatpanic", database: "ecto_simple", password: "supersecret"], seqnum: 0, sock: {Mariaex.Connection.Tcp, #Port<0.5433>}, ssl_conn_state: :not_used, state: :running, state_data: nil, timeout: 5000})
        (db_connection) lib/db_connection/connection.ex:373: DBConnection.Connection.handle_info/2
        (connection) lib/connection.ex:810: Connection.handle_async/3
        (stdlib) gen_server.erl:616: :gen_server.try_dispatch/4
        (stdlib) gen_server.erl:686: :gen_server.handle_msg/6
        (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
        
Interactive Elixir (1.6.1) - press Ctrl+C to exit (type h() ENTER for help)
iex(5)> 
21:17:06.515 [error] GenServer #PID<0.180.0> terminating
** (FunctionClauseError) no function clause matching in Mariaex.Protocol.ping_handle/3
    (mariaex) lib/mariaex/protocol.ex:1139: Mariaex.Protocol.ping_handle({:packet, 30, 0, {:error_resp, 255, 1927, "#", "70100", "Connection was killed"}, <<255, 135, 7, 35, 55, 48, 49, 48, 48, 67, 111, 110, 110, 101, 99, 116, 105, 111, 110, 32, 119, 97, 115, 32, 107, 105, 108, 108, 101, 100>>}, :ping, %Mariaex.Protocol{binary_as: :field_type_var_string, buffer: "", cache: #Reference<0.2383522203.2188771331.72263>, catch_eof: false, connection_id: #PID<0.180.0>, cursors: %{}, deprecated_eof: true, json_library: Poison, lru_cache: {100, #Reference<0.2383522203.2188771331.72267>}, opts: [socket_options: [], sock_type: :tcp, cache_size: 100, timeout: 5000, port: 3306, hostname: "localhost", username: "beatpanic", database: "ecto_simple", password: "supersecret"], seqnum: 0, sock: {Mariaex.Connection.Tcp, #Port<0.5433>}, ssl_conn_state: :not_used, state: :running, state_data: nil, timeout: 5000})
    (db_connection) lib/db_connection/connection.ex:373: DBConnection.Connection.handle_info/2
    (connection) lib/connection.ex:810: Connection.handle_async/3
    (stdlib) gen_server.erl:616: :gen_server.try_dispatch/4
    (stdlib) gen_server.erl:686: :gen_server.handle_msg/6
    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Last message: :timeout
State: {Mariaex.Protocol, %Mariaex.Protocol{binary_as: :field_type_var_string, buffer: "", cache: #Reference<0.2383522203.2188771331.72263>, catch_eof: false, connection_id: #PID<0.180.0>, cursors: %{}, deprecated_eof: true, json_library: Poison, lru_cache: {100, #Reference<0.2383522203.2188771331.72267>}, opts: [socket_options: [], sock_type: :tcp, cache_size: 100, timeout: 5000, port: 3306, hostname: "localhost", username: "beatpanic", database: "ecto_simple", password: "supersecret"], seqnum: 0, sock: {Mariaex.Connection.Tcp, #Port<0.5433>}, ssl_conn_state: :not_used, state: :running, state_data: nil, timeout: 5000}}

[kpanic]

Closing the issue since the related PR was merged