Improve parser error messaging when processing parent/plugin

[mmulholla]

My test has a devfile which specifies a parent uri, but I had made a typo in the uri.

In this case the parser, when asked to process the devfile, gives this error: "apiVersion or schemaVersion not present in devfile"

This happened because when the parser reads the parent it assume the target uri exists and can't find a schema version.

It would have been very helpful had the message been:
preferred: that the parent uri was not valid
or: that the schema version was not found in the parent.