Fix SF #1117398, cookielib LWPCookieJar and MozillaCookieJar exceptions

cookielib.LWPCookieJar and .MozillaCookieJar are documented to raise
cookielib.LoadError on attempt to load an invalid cookies file, but
raise IOError instead.  Compromise by having LoadError subclass IOError.

Lib/test/test_cookielib.py

@@ -248,6 +248,31 @@ class FileCookieJarTests(TestCase):
             except OSError: pass
         self.assertEqual(c._cookies["www.acme.com"]["/"]["boo"].value, None)
 
+    def test_bad_magic(self):
+        from cookielib import LWPCookieJar, MozillaCookieJar, LoadError
+        # IOErrors (eg. file doesn't exist) are allowed to propagate
+        filename = test_support.TESTFN
+        for cookiejar_class in LWPCookieJar, MozillaCookieJar:
+            c = cookiejar_class()
+            try:
+                c.load(filename="for this test to work, a file with this "
+                                "filename should not exist")
+            except IOError, exc:
+                # exactly IOError, not LoadError
+                self.assertEqual(exc.__class__, IOError)
+            else:
+                self.fail("expected IOError for invalid filename")
+        # Invalid contents of cookies file (eg. bad magic string)
+        # causes a LoadError.
+        try:
+            f = open(filename, "w")
+            f.write("oops\n")
+            for cookiejar_class in LWPCookieJar, MozillaCookieJar:
+                c = cookiejar_class()
+                self.assertRaises(LoadError, c.load, filename)
+        finally:
+            try: os.unlink(filename)
+            except OSError: pass
 
 class CookieTests(TestCase):
     # XXX