Manual notify exception

[mcass19]

This PR aims to implement the ability to manually send an exception. It's a first try so feel free to discuss / suggest whatever you want. Actually, there are a few things to discuss to set a direction to go.

To do / discuss

• The manual exceptions are always sending an empty stacktrace for now
• I added an extra check to see if we are on the router when defining handle_errors. I don't like it too much but I think it covers most of the cases (it doesn't cover if the user defines a module ending in .Router)
• I changed the configuration so that it is read from the application environment, since now we can do use BoomNotifier anywhere and with the current implementation it would be necessary to always write it
  • But maybe the user wants different settings in different places or am I crazy?
  • Update tests if we keep this change
  • Update readme if we keep this change
• Do we want to always send the conn object or do we want to be more open to any elixir code and just send the exception without much more information?
• I defined a new manual_notify_error callback, but maybe it is better to have only one notify_error that handles all cases?

[jmbejar]

• The manual exceptions are always sending an empty stacktrace for now

I think we can send (or teach the user to send) the stacktrace from the rescue block, using STACKTRACE/0.

[jmbejar]

I changed the configuration so that it is read from the application environment, since now we can do use BoomNotifier anywhere and with the current implementation it would be necessary to always write it

• But maybe the user wants different settings in different places or am I crazy?

I think it is fair to have a unique global configuration, so we have no other choice :)

[jmbejar]

I feel this should be an import instead of a use. So, you can avoid all the extra complexity of checking if we are in the context of a router or a controller in BoomNotifier.__using__.

In fact, it is probably fine to say that the user should invoke BoomNotifier.notify_error directly (the user can or not add an import)

[jmbejar]

Well, the only thing to solve here is to make sure that config validations runs at least once. If the module is used in the Router is fine, but we should make it mandatory (even if it is not going to be used there).

Another option would be to check configs directly in the new Config module, as part of any of the functions defined there.. or maybe as an operation that is done only once and cached.

[jmbejar]

What about using the already existing notify_error function? I think all the logic there applies here (limits & filters for exceptions). So, we would have similar ways in router and controller to notify errors when exceptions are being intercepted by user code (error_handler or simple try-catch-rescue blocks).