Include reason for validator failure in Cookie errorContext

With the introduction of errorContext (MB-23909, added in 5.0.0) we have the ability to include an additional contextual error string in response packets.

Given that some mcbp requests have quite complex encoding rules (e.g. Subdoc, particulary multi-ops) it would assist in debugging invalid requests if we made use of Cookie::setErrorContext() to provide more details on exactly why a request failed.

To give an example; see subdoc_validator - this has a relatively complex sequence of checks which a sub-document request must pass to be considered valid; if it is not valid then (virtually) all return code are EINVAL with no additional context. Improve this by adding explicit descriptions of why each check failed, and expand the corresponding subdoc tests to check for the correct error context string.