afs: Pay attention to fetchOps->destroy error code

The ->destroy function in our fetchops could change our error code, or
even raise a new error. Don't ignore it. This currently doesn't do
much, since fetchDestroy currently won't change the error code if it's
given an error, but this can change in the future.

Change-Id: I6fa98cc709cb0fbd4c1e868ba4b9be53313573ff
Reviewed-on: http://gerrit.openafs.org/10787
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>