error message is ambiguous when fusermount is missing

[ubnt-intrepid]

... such as

Error: too short control message length

It was reported in https://github.com/ubnt-intrepid/polyfuse/issues/83#issuecomment-770389984.

[ubnt-intrepid]

The problem here is that the error code caused by exec(2) in the forked process will not reported to the parent process.