Refactor artifact cache exceptions.
Review Request #1261 — Created Oct. 31, 2014 and submitted
|benjyw, davidt, ity, johanoskarsson|
Presently, the artifact cache will treat any exception during the insertion process as a non-fatal error. But some errors should be fatal -- any sort of misconfiguration, or an attempt to cache nonexistent files, or an exception thrown while moving files around. Also remove ArtifactCache.CacheError, as it was not clear when to use ArtifactCache.CacheError and when to use ArtifactCacheError.