cli: if directories do not exist, `ipfs mount` returns ugly error

[jbenet]

Is:

Error: fusermount: "fusermount: failed to access mountpoint /ipfs: No such file or directory\n", exit status 1

Should be

Error: failed to access mountpoint /ipfs: No such file or directory
USAGE
 ...
 ...

[jbenet]

Fixed in: 981c5ab