show an error page on 500.

[fiatjaf]

For a long time we've had a "500.html" template that seated uselessly in the codebase. Not anymore!

Changes proposed in this pull request:

• Add a Flask handler for exceptions that would generate an HTTP 500 and show a better page to the user with a stacktrace that may be used for debugging.
• Log these exceptions in a succinct way (using http://structlog.readthedocs.io/en/stable/api.html#structlog.processors.ExceptionPrettyPrinter).

Have you made sure to add:

• Tests
• Documentation

Screenshots and GIFs

[rohitdatta]

I think that instead of just showing the stack trace, we should generate a unique ID, place on the page the current time and unique ID, with a request for them to email us these pieces of information.

We then put the unique ID and stack trace into the logs to avoid scaring the user (which could not even be the original site developer but a user) with a stack trace.