### Please describe the problem.
This is less of a bug and more of a feature(?) request. Currently, when assigning metadata to a datafile, git annex (v5.20150710-g8fd705) will produce no error or warning message if the file entered does not exist. This can be a tad confusing to users who might expect to see some output if they typed their path wrong. A successful `git annex metadata <filename> -s tags=best` will produce output acknowledging the change, but a failure produces no output at all. A quick check if the file exists, and a `File does not exist` error message if not, would be easy implement and helpful to new users.

### What steps will reproduce the problem?

git annex metadata <filename> -s tags+=atlas

Where <filename> does not exist.

### What version of git-annex are you using? On what operating system?

v5.20150710-g8fd705

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

We use git-annex for our open-source !FreeSurfer software and find very helpful indeed. Thank you. https://surfer.nmr.mgh.harvard.edu/

> [[fixed|done]] --[[Joey]]
