frontend: exception handlers fix once more
Previous variant entirely swallowed the tracebacks, which are needed in
logs to easily see and fix the problems. Fix this.
Also, re-type the CoprHttpException.message to string early. This fixes
traceback in API BadRequest handler:
File "/usr/lib/python3.9/site-packages/flask/app.py", line 2464, in __call__
return self.wsgi_app(environ, start_response)
File "/usr/lib/python3.9/site-packages/flask/app.py", line 2450, in wsgi_app
response = self.handle_exception(e)
File "/copr/coprs_frontend/coprs/rest_api/__init__.py", line 44, in error_router
return original_handler(e)
File "/usr/lib/python3.9/site-packages/flask/app.py", line 1867, in handle_exception
reraise(exc_type, exc_value, tb)
File "/usr/lib/python3.9/site-packages/flask/_compat.py", line 39, in reraise
raise value
File "/usr/lib/python3.9/site-packages/flask/app.py", line 2447, in wsgi_app
response = self.full_dispatch_request()
File "/usr/lib/python3.9/site-packages/flask/app.py", line 1952, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/copr/coprs_frontend/coprs/rest_api/__init__.py", line 44, in error_router
return original_handler(e)
File "/usr/lib/python3.9/site-packages/flask/app.py", line 1822, in handle_user_exception
return handler(e)
File "/copr/coprs_frontend/coprs/__init__.py", line 149, in handle_exceptions
return error_handler.handle_error(error)
File "/copr/coprs_frontend/coprs/error_handlers.py", line 88, in handle_error
message = self.message(error)
File "/copr/coprs_frontend/coprs/error_handlers.py", line 55, in message
return str(error)