Handle Invalid GIT_EDITOR

[wookie184]

Noticed because of this: microsoft/vscode#153246

This error is raised when running blurb with an invalid GIT_EDITOR environment variable ("c:\Users\wookie184\AppData\Local\Programs\Microsoft VS Code Insiders\resources\app\extensions\git\dist\git-editor.sh" on windows in my case).

PS C:\Users\wookie184\Documents\GitHub\cpython> blurb
Traceback (most recent call last):
  File "c:\users\wookie184\appdata\local\programs\python\python39\lib\runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "c:\users\wookie184\appdata\local\programs\python\python39\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "C:\Users\wookie184\AppData\Local\Programs\Python\Python39\Scripts\blurb.exe\__main__.py", line 7, in <module>
  File "c:\users\wookie184\appdata\local\programs\python\python39\lib\site-packages\blurb.py", line 1659, in main
    sys.exit(fn(*filtered_args, **kwargs))
  File "c:\users\wookie184\appdata\local\programs\python\python39\lib\site-packages\blurb.py", line 935, in add
    subprocess.run(args)
  File "c:\users\wookie184\appdata\local\programs\python\python39\lib\subprocess.py", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "c:\users\wookie184\appdata\local\programs\python\python39\lib\subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "c:\users\wookie184\appdata\local\programs\python\python39\lib\subprocess.py", line 1420, in _execute_child
    hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
OSError: [WinError 193] %1 is not a valid Win32 application

It isn't very clear to the user what the issue is from this error. Possible solutions:

• Handle the error and reraise a specific error mentioning what failed to run (e.g. that GIT_EDITOR was invalid if that is the case).
• Handle the error and try to use a fallback (e.g. notepad on windows) if GIT_EDITOR fails.

If we decide to do one of these I'd be happy to try implementing it.

Thanks!