When no default editor is installed and no environment variable is set,
which_string would return None and this would be passed to os.execv
resulting in a TypeError. The message presented to the user would be:
Error: execv: path should be string, bytes or os.PathLike,
not NoneType
This change checks that which_string has returned successfully before
attempting to execute the result, resulting in a new error message:
Error: No text editor found! Please set the VISUAL and/or EDITOR
environment variable(s) to your preferred text editor.
It's not strictly necessary, but I've also changed try_exec to catch
all errors rather than just OSErrors. This would have provided slightly
more context for the original error message.
bb92ea59a2