Convert any ChaliceViewError to a response

[vfrico]

Description of changes:
Using a method on the class ChaliceViewError would allow any subclass to implement its own custom response for an exception.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jamesls]

Hi, thanks for the pull request, seems reasonable to me. I'd like to get a test added to tests/unit/test_app.py that verifies this behavior with a custom subclass to ensure we don't regress on this behavior in the future.

[vfrico]

I've just added tests and docs.

On my local machine, the command make prcheck runs ok, but on the last commit the tests didn't ran successfully.