Make db modules' error classes inherit IOError.
Stop dbm from importing every dbm module when imported.
diff --git a/Lib/dbm/bsd.py b/Lib/dbm/bsd.py
index 8353f50..2dccadb 100644
--- a/Lib/dbm/bsd.py
+++ b/Lib/dbm/bsd.py
@@ -4,7 +4,8 @@
 
 __all__ = ["error", "open"]
 
-error = bsddb.error
+class error(bsddb.error, IOError):
+    pass
 
 def open(file, flag = 'r', mode=0o666):
     return bsddb.hashopen(file, flag, mode)