Add error message when repo is not reachable

Currently git:check (invoked by deploy:check) does not print anything when `git ls-remote` is failed and **silently** aborted. It would be better to show some error messages when it is aborted.

Output when log level is info (current):
```
 INFO [f20ac338] Running /usr/bin/env mkdir -p /tmp/foobar/ on example.com
 INFO [f20ac338] Finished in 0.225 seconds with exit status 0 (successful).
 INFO Uploading /tmp/foobar/git-ssh.sh 100.0%
 INFO [8f04ecda] Running /usr/bin/env chmod +x /tmp/foobar/git-ssh.sh on example.com
 INFO [8f04ecda] Finished in 0.014 seconds with exit status 0 (successful).
```

Output when log level is debug  (current):
```
 INFO [55aa79d3] Running /usr/bin/env mkdir -p /tmp/foobar/ on example.com
DEBUG [55aa79d3] Command: /usr/bin/env mkdir -p /tmp/foobar/
 INFO [55aa79d3] Finished in 0.208 seconds with exit status 0 (successful).
DEBUG Uploading /tmp/foobar/git-ssh.sh 0.0%
 INFO Uploading /tmp/foobar/git-ssh.sh 100.0%
 INFO [ee1464f1] Running /usr/bin/env chmod +x /tmp/foobar/git-ssh.sh on example.com
DEBUG [ee1464f1] Command: /usr/bin/env chmod +x /tmp/foobar/git-ssh.sh
 INFO [ee1464f1] Finished in 0.008 seconds with exit status 0 (successful).
DEBUG [c6b76799] Running /usr/bin/env git ls-remote git@github.com:foo/foobar.git on example.com
DEBUG [c6b76799] Command: ( GIT_ASKPASS=/bin/echo GIT_SSH=/tmp/foobar/git-ssh.sh /usr/bin/env git ls-remote git@github.com:foo/foobar.git )
DEBUG [c6b76799] 	/usr/bin/env:
DEBUG [c6b76799] 	git
DEBUG [c6b76799] 	: No such file or directory
DEBUG [c6b76799]
DEBUG [c6b76799] Finished in 0.213 seconds with exit status 127 (failed).
```