mount: propagate error codes correctly

Make sure to propagate error codes from mount-loops correctly. Right now,
we return the return-code of the first mount that did _something_. This is
not what we want. Make sure we return an error if _any_ mount fails (and
then make sure to return the first error to not hide proper errors due to
consequential errors like -ENOTDIR).

Reported by cee1 <fykcee1@gmail.com>.