decrement mount count if unmount fails

[navinms711]

Summary

This PR addresses the issue describe in https://vmw-jira.broadcom.net/browse/TNZ-71700. Following changes are made:

(1) One new custom error type MountPointNotExistError for wrapping os.ErrNotExist() error.
(2) Two new private methods removeMountPath() and decrementMountCount on VolumeDriver. This removes code duplication in many places.
(3) Will any of the callers fail because of error wrapping? No.
(4) New unit tests are added.

Summary of Error code Paths
-------------------------------------------------------------------------------------------------------
Error Path                              Mount Count decremented ?      Error Preserved ?
                                        BEFORE  |   AFTER              BEFORE    |    AFTER
-------------------------------------------------------------------------------------------------------
Validation errors                        No     |   No                 n/a       |    n/a
mountChecker.Exists()                    No     |   Yes                Yes       |    Yes
os.Remove() -> os.ErrNotExist            No     |   Yes                Yes       |    Yes  (wrapped)
os.Remove() -> other errors              No     |   Yes                Yes       |    Yes  (wrapped)
Mount not exist, remove succeeds         No     |   Yes                n/a       |    n/a
mounter.Unmount()                        No     |   Yes                No        |    Yes  (wrapped)
persistState()                           No     |   Yes                No        |    No
--------------------------------------------------------------------------------------------------------

Backward Compatibility

Yes, backward compatible.

[mariash]

LGTM