improve logging when blob is not found: include the filename requested

  which doesn't exists

CHANGES.rst

@@ -2,6 +2,12 @@
  Change History
 ================
 
+4.0.0.dev (unreleased)
+======================
+
+- improve logging when blob is not found: include the filename requested
+  which doesn't exists
+
 4.0.0a4 (2012-12-17)
 =====================
 

src/ZODB/blob.py

@@ -642,7 +642,7 @@ class BlobStorageMixin(object):
         """
         filename = self.fshelper.getBlobFilename(oid, serial)
         if not os.path.exists(filename):
-            raise POSKeyError("No blob file", oid, serial)
+            raise POSKeyError("No blob file at %s" % filename, oid, serial)
         return filename
 
     def openCommittedBlobFile(self, oid, serial, blob=None):