Allow errors to be swallowed in error hooks

[JaneOri]

For #151

This is (mostly) just copying the necessary parts and test from @daffl's fix found in the main project:
https://github.com/feathersjs/feathers/pull/621/files

Use case:

I have a create user (sign up) service that returns just the email that was entered and we show a message that says something like Email sent! Please check ${response.email} for additional information.

However, if the user already exists then the service returns an error - which is good if the server or an admin user runs it.

BUT if the request is from a non-admin end user, we need to return the original non-error reply as if the email didn't exist. (so we don't leak information to anyone maliciously using the form about who's registered)

In order to do that in a clean way, we need to conditionally swallow the error in an error hook.

Breaking change:

If hook.result is set and then an error happens, hook.result will now be null. The original result can still be accessed from hook.original.result.

[daffl]

Thank you James. Released as 2.1.0

[marshallswain]

Thanks @daffl

[FossPrime]

Seems like setting hook.result in a find is not good enough to pass the hook validation step... ideas? get error hooks work great however.