Automatic merge from submit-queue

Fix GCE attacher/detacher to ignore return value of failed calls.

The plugin should ignore any return value if err is set. Found when writing unit tests in #26615 - my dummy `DiskIsAttached` returned `false, errors.New('fake error')` and the volume was **not** detached although the log message `"Error checking if PD (%q) is already attached to current node (%q). Will continue and try detach anyway."` suggested otherwise 

@saad-ali, PTAL
@kubernetes/sig-storage