Raise unversioned error when config not allowed

[davidlatwe]

Problem

When building unversioned package with config.allow_unversioned_packages = False, the error raised and says :

  ...
  File "..\rezplugins\package_repository\filesystem.py", line 715, in _create_variant
    overrides=overrides
  File "..\rezplugins\package_repository\filesystem.py", line 1228, in _create_variant
    raise RezSystemError("Internal failure - expected installed variant")
rez.exceptions.RezSystemError: Internal failure - expected installed variant

Which isn't clear enough to know that the failure was due to building a package which is unversioned while Rez has been configured to not allow it.

Propose

Check package data and config settings between data override applying and data dumping.
And the output would be :

Invoking custom build system...
20:48:55 ERROR    PackageMetadataError: Unversioned package is not allowed in current configuration.

[nerdvegas]

Thanks again for yet another concise and explanatory PR!