fixup! Handle NotImplementedError raised by loadBefore/loadBeforeEx as "interface not provided"

loadBeforeEx uses the same docstring as loadBefore as @d-maurer suggests:

https://github.com/zopefoundation/ZODB/pull/323#discussion_r626255153

src/ZODB/BaseStorage.py

@@ -272,8 +272,8 @@ class BaseStorage(UndoLogCompatible):
         """Return most recent revision of oid before tid committed."""
         raise NotImplementedError
 
-    def loadBeforeEx(self, oid, before):
-        """Return most recent revision of oid as of <before database state.
+    def loadBeforeEx(self, oid, tid):
+        """Return most recent revision of oid before tid committed.
            (see IStorageLoadBeforeEx).
         """
         raise NotImplementedError