Allow on_cleanup_error callback to suppress exceptions

[rittneje]

Currently we can set context.on_cleanup_error to suppress certain exceptions from being printed to the console. However, even if we do this, behave will re-raise the exception, and ultimately cause the scenario/feature/run to be marked as a failure. We can disable this behavior via Context.FAIL_ON_CLEANUP_ERRORS and/or context.fail_on_cleanup_errors = False but this is a very broad switch. We can also do try/except ourselves but that kind of defeats the point of on_cleanup_error.

Instead, it would be nice if on_cleanup_error could return a boolean indicating that the exception in question can be ignored. For backwards compatibility, if it returns None then the exception is not dropped.