CA-297698 improve assert message

Add some additional information to an extremely rare assert, to make it
a bit easier to work out what's going on if it ever happens again.

drivers/blktap2.py

@@ -1187,7 +1187,7 @@ def mklink(self, target):
                 # idempotent. However, a *wrong* target should never
                 # be seen.
                 if e.errno != errno.EEXIST: raise
-                assert self._equals(target)
+                assert self._equals(target), "'%s' not equal to '%s'" % (path, target)
 
         def unlink(self):
             try: