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.

lib/spack/spack/test/util/editor.py

@@ -132,3 +132,17 @@ def assert_exec(exe, args):
         assert args == [vim_exe, '/path/to/file']
 
     ed.editor('/path/to/file', _exec_func=assert_exec)
+
+
+def test_no_editor():
+    if 'VISUAL' in os.environ:
+        del os.environ['VISUAL']
+    if 'EDITOR' in os.environ:
+        del os.environ['EDITOR']
+    os.environ['PATH'] = ''
+
+    def assert_exec(exe, args):
+        assert False
+
+    with pytest.raises(EnvironmentError, match=r'No text editor found.*'):
+        ed.editor('/path/to/file', _exec_func=assert_exec)

lib/spack/spack/util/editor.py

@@ -122,7 +122,7 @@ def try_env_var(var):
     # trying them all -- if we get here and one fails, something is
     # probably much more deeply wrong with the environment.
     exe = which_string(*_default_editors)
-    if try_exec(exe, [exe] + list(args)):
+    if exe and try_exec(exe, [exe] + list(args)):
         return
 
     # Fail if nothing could be found