More tags when logging errors to Sentry

[humitos]

This helps us to filter different kind of random/unhandled errors by tags and make debugging easier.

[humitos]

I think that we could extend from SentryHandler and write our custom one that depending on the context adds these tags automatically before this line https://github.com/getsentry/raven-python/blob/master/raven/handlers/logging.py#L97 by modifying the record object.

This way, we don't need to add tags all around the project's source code and we could guess where we are or what type of error are we handling (build, web, etc) and add the corresponding tags for each case. Just an idea, though.

[davidfischer]

This makes sense to me and looks good.

I do have some concerns about the default value of self.project and self.build being None and then we assume it is set correctly but that is probably best for a different PR.