Properly handle references to missing schema files

[kevin-bates]

References to "missing" schema (which can include typos in the schema_name value) were not handled correctly, leading to an unhandled exception. These changes include such occurrences as invalid metadata and will include such results upon request (and not be unhandled). The custom-defined errors now derive from Exception rather than BaseException to accommodate this change - which is probably better anyway.

Note since issue #1093 was not marked for elyra 2.x, I'm including it in master for now. These changes should easily migrate to the elyra 2.x branch when that is officially built.

Fixes #1093

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