[frontend] rework error handlers to fix #531
There are multiple issues with our current implementation
of error handling.
First, `apiv3_ns` defines error handlers on application level,
not blueprint level. This causes the #531 issue and results in
showing API errors even for exceptions, that happened in UI.
Also, `coprs_ns` defines error handler for 404, but only for the case,
that some object is not found. It doesn't cover the case when
a whole API endpoint is not found. We can't add it there, though.
According to the Flask documentation
http://flask.pocoo.org/docs/1.0/blueprints/#error-handlers
it is not possible to add handlers for 404 and 405 on blueprint level.
That's why it is currently implemented in `misc.py`.
That basically means, that our error handlers are inconsistently spread
within the application and we need to have some of them for `apiv3_ns`
and `coprs_ns` blueprints and also a conditioned handler for 404 on
application level. I have had multiple moments of clarity, thinking
that I finally understand how the code works, but I didn't. And then
again and gain, so I decided to rather have all handlers implemented
the same way.