Better handling of internal errors.

1. Attempt to report internal errors as exceptions to Rollbar. Internal errors are reported in the normally-configured project (for examination by the project developers) and flagged as internal (for examination by Rollbar staff).
2. if reporting the internal error fails, send a mostly-static failsafe response. The failsafe is also reported in the project context, and flagged as both "internal" and "failsafe".
3. if the failsafe fails, log an error.

Additionally: all logger calls in lib/rollbar.rb are now done via wrappers (`log_error`, etc.) that catch exceptions in such calls.