[Proposal] Allow for registration of custom error handlers

[cloutierMat]

I first put my proposal in response to #1884. But in an effort to make sure the work I do is in a desired direction, I am copying my comments here so that it can be seen and hopefully handled.

Problem

While the ChaliceUnhandledError works fine for custom errors, third party libraries can raise Exceptions that can't be subclassed. Having a try/except in a middleware won't successfully catch the Exception as it is already a Response at that point.

Goals

• Exceptions on REST APIs must be intercepted before the InternalServerError response is built.
• Must have knowledge of the context of the raised exception.
• Must be able to return a Response
• An invalid Response or an exception raised in the handler should still return the standard InternalServerError and have the stack trace replaced by the expected json object when not in debug mode
• As existing middlewares can already successfully intercept all exceptions for other events, and ChaliceUnhandledError only exists in the scope of http event, so will the focus of this handler.

Specification

A new @app.errorhandler is to be added. The decorated function will take one arg, the exception.

Similar to the way middlewares can be registered, we will be able to register the error handler in app or in a blueprint with either a decorator or app.register_error.

# Decorator
@app.errorhandler(MyCustomError)
def my_handler(e: MyCustomError):
    return Response(body="", status_code:400)

# Register
app.register_error_handler(MyCustomError, my_handler)

# Blueprint
@my_blueprint.errorhandler(MyCustomError)
def my_handler(e: MyCustomError):
    return Response(body="", status_code:400)
In the examples above, any http endpoint raising MyCustomError will then return a 400 with no body.

If, however, the return value isn't a Response or an exception was raised during execution of the handler, the normal flow of the RestAPIEventHandler will be maintained.

@app.errorhandler(MyCustomError)
def my_handler(e: MyCustomError):
    return "Not a Response object"

Will return

{
    "Code": "InternalServerError",
    "Message": "An internal server error occurred."
}