[ORC] Consolidate materialization errors, and generate them in VSO's

notifyFailed method rather than passing in an error generator.

VSO::notifyFailed is responsible for notifying queries that they will not
succeed due to error. In practice the queries don't care about the details
of the failure, just the fact that a failure occurred for some symbols.
Having VSO::notifyFailed take care of this simplifies the interface.

llvm-svn: 332666