Make UnravelError exception a little more helpful

Author: srynot4sale

src/saml2/entity.py

@@ -394,7 +394,7 @@ def unravel(txt, binding, msgtype="response"):
                 else:
                     xmlstr = txt
             except Exception:
-                raise UnravelError()
+                raise UnravelError("Unravelling binding '%s' failed" % binding)
 
         return xmlstr