Handle incompatible values for DataObjectHandleBase props

Backport of !650 (merged)

GaudiKernel/python/GaudiKernel/PropertyProxy.py

@@ -385,6 +385,9 @@ class DataObjectHandleBasePropertyProxy(PropertyProxy):
             return DataObjectHandleBase(value)
         elif isinstance(value, DataObjectHandleBase):
             return DataObjectHandleBase(value.__str__())
+        else:
+            raise ValueError("received an instance of %s, but %s expected" %
+                             (type(value), 'str or DataObjectHandleBase'))
 
 
 def PropertyProxyFactory(descr, doc, default):