Descriptive errors

Some common, very specific errors give vague messages.
One in particular was caught a few times during the BrainHack workshop.

When a `gin get` is called and the directory already exists, the command fails with a vague error message. It would be trivial to distinguish this case and give an informative error message.

There are other errors like this that could use work.

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**UPDATE**
Checklist of errors to handle:
- [x] Directory already exists
- [x] Permission denied on git command
- [x] File not found (e.g., on `gin get-content`)
- [x] Unable to unlock file with no content
- [x] Unable to lock, unlock, get-content, or remove-content of untracked or non-annexed files
- [x] Unable to run any git commands outside a git repository
- [x] Unable to run any command on file that does not exist
- [ ] Not enough free space (git annex performs the check itself. Related issue #21)