Automatic merge from submit-queue (batch tested with PRs 62324, 61459, 62475, 62476, 61914). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Return error in mount_unsupported for unsupported platforms

**What this PR does / why we need it**:

Presently, `util/mount_unsupported.go` does not return any errors. For unsupported platforms, this hides failures. This PR returns errors, thereby properly informing users attempting to run on an unsupported platform.

**Which issue(s) this PR fixes** :
Fixes https://github.com/kubernetes/kubernetes/issues/61919

**Special notes for your reviewer**:

There are a few methods that simply call through to other methods. I did not return errors from those methods.

I've also updated an error check and message in `volume/fc/fc_test.go`, since it was ignoring an error on unsupported platforms.

**Release note**:
```release-note
NONE
```