Fix spack edit message when no editor installed (#31412)

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.

0 files changed, 0 insertions, 0 deletions