_artifactcache.py: Raise ArtifactError() when quota size exceeds disk space.

This is not an error related to loading data, like a parse error
in the quota specification is, but a problem raised by the artifact
cache - this allows us to assert more specific machine readable
errors in test cases (instead of checking the string in stderr, which
this patch also fixes).

This also removes a typo from the error message in the said error.

  * tests/artifactcache/cache_size.py

    Updated test case to expect the artifact error, which consequently
    changes the test case to properly assert a machine readable error
    instead of asserting text in the stderr (which is the real, secret
    motivation behind this patch).

  * tests/artifactcache/expiry.py: Reworked test_invalid_cache_quota()

    Now expect the artifact error for the tests which check configurations
    which create caches too large to fit on the disk.