Updated a variety of language and error messages

In api/client.go updated:

1. Made api == API
2. Restated Docker daemon connection error to be clearer.
3. Changed use of "Impossible" in error messages.
4. Removed extra space in restart error message.
5. Fixed capitalization in three error messages.

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)