Retry on "Connection broken:" errors

[bdice]

Closes #98.

This PR adds a rapids-conda-retry mode for errors that include the string "Connection broken:".

This includes errors like the following:

("Connection broken: ConnectionResetError(104, 'Connection reset by peer')", ConnectionResetError(104, 'Connection reset by peer'))
("Connection broken: ConnectionResetError(104, 'Connection reset by peer')", ConnectionResetError(104, 'Connection reset by peer'))

and

('Connection broken: IncompleteRead(584079781 bytes read, 131525879 more expected)', IncompleteRead(584079781 bytes read, 131525879 more expected))
('Connection broken: IncompleteRead(584079781 bytes read, 131525879 more expected)', IncompleteRead(584079781 bytes read, 131525879 more expected))

Also, I alphabetized the error modes to make it easier to read.