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

Suggested by @d-maurer: https://github.com/zopefoundation/ZODB/pull/323#discussion_r625573381

src/ZODB/BaseStorage.py

@@ -268,8 +268,15 @@ class BaseStorage(UndoLogCompatible):
         raise POSException.Unsupported(
             "Retrieval of historical revisions is not supported")
 
-    # do not provide loadBeforeEx/loadBefore here in BaseStorage - if child forgets
-    # to override it - storage will always return "no data" instead of failing.
+    def loadBefore(self, oid, tid):
+        """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.
+           (see IStorageLoadBeforeEx).
+        """
+        raise NotImplementedError
 
     def copyTransactionsFrom(self, other, verbose=0):
         """Copy transactions from another storage.

src/ZODB/utils.py

@@ -397,7 +397,10 @@ def loadBeforeEx(storage, oid, before):
     """
     loadBeforeEx = getattr(storage, 'loadBeforeEx', None)
     if loadBeforeEx is not None:
-        return loadBeforeEx(oid, before)
+        try:
+            return loadBeforeEx(oid, before)
+        except NotImplementedError:
+            pass
 
     # storage does not provide IStorageLoadBeforeEx - warn + fall back to loadBefore
     if type(storage) not in _loadBeforeExWarned: