Raised SuspiciousFileOperation in safe_join.

Added a test for the condition safe_join is designed to prevent.

Previously, a generic ValueError was raised. It was impossible to tell
an intentional exception raised to implement safe_join's contract from
an unintentional exception caused by incorrect inputs or unexpected
conditions. That resulted in bizarre exception catching patterns, which
this patch removes.

Since safe_join is a private API and since the change is unlikely to
create security issues for users who use it anyway -- at worst, an
uncaught SuspiciousFileOperation exception will bubble up -- it isn't
documented.

tests/utils_tests/test_os_utils.py

@@ -1,6 +1,7 @@
 import os
 import unittest
 
+from django.core.exceptions import SuspiciousFileOperation
 from django.utils._os import safe_join
 
 
@@ -24,3 +25,7 @@ class SafeJoinTests(unittest.TestCase):
             path,
             os.path.sep,
         )
+
+    def test_parent_path(self):
+        with self.assertRaises(SuspiciousFileOperation):
+            safe_join("/abc/", "../def")