Handle Git command failures

[edudobay]

Cases to handle

• (Friendly error message when selecting a non-existing branch #33) Non-existing branch/commit/tag/ref

Example

When an invalid ref (e.g. non-existent branch) is given, we get this (solved by #33):

> Fetching from default remote
fatal: Needed a single revision
Traceback (most recent call last):
  File "/usr/local/bin/simple-git-deploy", line 11, in <module>
    sys.exit(main())
  File "/opt/pyenv/virtualenvs/tools/lib/python3.6/site-packages/simple_git_deploy/cli.py", line 52, in main
    args.func(args, config, reporter)
  File "/opt/pyenv/virtualenvs/tools/lib/python3.6/site-packages/simple_git_deploy/cmd_deploy.py", line 11, in cmd_deploy
    reporter=reporter
  File "/opt/pyenv/virtualenvs/tools/lib/python3.6/site-packages/simple_git_deploy/core.py", line 145, in prepare_new_tree
    hash = git_rev_parse_short(git_ref, git_dir)
  File "/opt/pyenv/virtualenvs/tools/lib/python3.6/site-packages/simple_git_deploy/core.py", line 122, in git_rev_parse_short
    encoding='utf-8',
  File "/opt/pyenv/versions/3.6.8/lib/python3.6/subprocess.py", line 356, in check_output
    **kwargs).stdout
  File "/opt/pyenv/versions/3.6.8/lib/python3.6/subprocess.py", line 438, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['git', 'rev-parse', '--short', 'hotfix/update_whippet_booking_to_order^{commit}']' returned non-zero exit status 128.

Instead we should get a friendly error message.