Catch more exceptions during boot process
Catch more exceptions during boot process

This extends our boot exception handling block to also catch and format
all exceptions that could be thrown while building our request handler,
i.e. the middleware stack handling requests.

The only exceptions that would now not be handled in this way could be
raised by Zend's `RequestHandlerRunner` and its delegates, which we
should be able to rely on.

Exceptions on request execution will be handled by the error handler in
the middleware stack.

Fixes #1607.

flarum/core,flarum/core,flarum/core