Use with lager via raven_lager_backend

[jtmoulia]

This may be outside of the scope of this repo, but this pull request brings raven-erlang and lager together.

I'm sure that more sentry friendly metadata can be extracted from lager, but before I drilled down I thought I'd see if this was even of interest.

Cool repo btw.

[asabil]

This is great! Are you interested in continuing the integration work to see if we could extract some metadata that could be pushed to sentry?

[jtmoulia]

Hey @asabil -- I definitely wouldn't mind! General metadata like the source PID and such should be easy, and I can follow raven_error_logger's lead.

One issue: I'd like to avoid duplicating functionality: after issuing this pull request I found https://github.com/basho/lager/blob/7af433004d846f7489fab1f705f8b06f40171bb2/src/error_logger_lager_h.erl which performs for lager what raven_error_logger does for this repo -- handling and parsing events from the error_logger.

I think raven_error_logger works great, and makes more sense than raven_lager_backend re-parsing the errors after they pass through and are munged (though very prettily :) by lager. I'm actually not sure what the best approach would be for raven_lager_backend -- perhaps skipping these error_logger -> lager messages? though this seems inconsistent.

Hope that made sense -- thoughts?