Add an `error:` or `errors:` check

[peter-tomaselli]

It’d be good to have a JSONCheck where all it does is raise a message if the key error or errors appears in a response under test with a corresponding non-empty value. We could then use this for pseudo-monitoring of either live APIs, or APIs under development.

This new check could be opt-in and would not have to be on by default.

Right now, when an error suddenly appears in a response, the default checks only raise a warning, because

    > WARNING: The key errors exists in the value being tested, but not in the snapshot. Perhaps you need to update your snapshot?