Convert some old-style string exceptions to class exceptions.

Lib/test/test_rgbimg.py

@@ -4,7 +4,8 @@
 
 from test_support import verbose, unlink, findfile
 
-error = 'test_rgbimg.error'
+class error(Exception):
+        pass
 
 print 'RGBimg test suite:'
 

Lib/test/test_unpack.py

@@ -100,7 +100,8 @@ def __getitem__(self, i):
 
 # unpacking a sequence where the test for too long raises a different
 # kind of error
-BozoError = 'BozoError'
+class BozoError(Exception):
+    pass
 
 class BadSeq:
     def __getitem__(self, i):