Improve error reporting when "git" is not found

[barrywhart]

If git is not installed or isn't in the user's PATH, diff-quality fails with an error that non-technical (SQLFluff, other?) users find confusing:

$ diff-quality --violations sqlfluff
Traceback (most recent call last):
  File "/app/.venv/bin/diff-quality", line 8, in <module>
    sys.exit(main())
  File "/app/.venv/lib/python3.9/site-packages/diff_cover/diff_quality_tool.py", line 300, in main
    GitPathTool.set_cwd(directory)
  File "/app/.venv/lib/python3.9/site-packages/diff_cover/git_path.py", line 33, in set_cwd
    cls._root = cls._git_root()
  File "/app/.venv/lib/python3.9/site-packages/diff_cover/git_path.py", line 65, in _git_root
    git_root = execute(command)[0]
  File "/app/.venv/lib/python3.9/site-packages/diff_cover/command_runner.py", line 27, in execute
    process = subprocess.Popen(command, stdout=stdout_pipe, stderr=stdout_pipe)
  File "/usr/local/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/local/lib/python3.9/subprocess.py", line 1821, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'git'

Perhaps it could catch this error and print a message something like:

<<command name>> requires git to be installed and in your PATH, but it was not found.

It could also link to this page, which has instructions for installing git on various OSes.

https://git-scm.com/book/en/v2/Getting-Started-Installing-Git