Returning custom error while unmounting /dev/fd/N kind of mount point

[raj-prince]

Issue:

• Misleading error logs while unmount GCSFuse in GKE world, where mount is created externally and passed via GCSfuse in /dev/fd/N format.

"Failed to unmount in response to SIGTERM: exec: \"fusermount\": executable file not found in $PATH

• This happens, as jacobsa/fuse library tries to unmount using fusermount binary, which is not present in GKE container environment and leads to failure.

Proposal:

• Return a custom error during fuse.Unmount() so that file-system implementation can handle appropriately.

Testing:
Working as expected.