Issue #14285: Do not catch exceptions initializing any ancestor package

The previous fix only handled the case of the parent package of __main__
failing to initialize.

Also make the "Error while finding spec" formatting slightly more appealing,
and document and test that the module name must be absolute.

Lib/test/test_cmd_line_script.py

@@ -433,6 +433,7 @@ class CmdLineTest(unittest.TestCase):
             ('importlib', br'No module named.*'
                 br'is a package and cannot be directly executed'),
             ('importlib.nonexistant', br'No module named'),
+            ('.unittest', br'Relative module names not supported'),
         )
         for name, regex in tests:
             with self.subTest(name):