Store exceptions in request env

[travisofthenorth]

I know that there is an exception whitelist, and that all exceptions can be whitelisted in the latest version with the whitelist_all_exceptions configuration. IMO, this is tangential issue, because as a developer, I might want the gem to handle an exception for me as far as responding to users with error messages, but I also want this exception to be logged separately.

For example, I rely on a Rollbar middleware to report exceptions for alerting/monitoring of my services. Because the gem was swallowing exceptions (in this case a database timeout, which should absolutely sound the alarms), I wasn't aware of a critical app error. Again, I still want the gem to handle responding to users with error messages, but I want those server errors to be processed by the rest of my middleware chain.

[travisofthenorth]

For reference, this is the Rollbar middleware I have in mind: https://github.com/rollbar/rollbar-gem/blob/master/lib/rollbar/middleware/rails/rollbar.rb#L26-L28

This is also what the ActionDispatch middleware sets, hence why Rollbar looks for it: https://github.com/rails/rails/blob/master/actionpack/lib/action_dispatch/middleware/show_exceptions.rb#L46

[HoyaBoya]

I think this is a great PR. 👍

[lgebhardt]

@travisofthenorth Thanks!