No bug - Improve error message when declaring an interface both as WebIDL and external in Bindings.conf; r=bz

dom/bindings/Bindings.conf

@@ -567,6 +567,8 @@ DOMInterfaces = {
 
 # These are temporary, until they've been converted to use new DOM bindings
 def addExternalIface(iface, nativeType=None, headerFile=None):
+    if iface in DOMInterfaces:
+        raise Exception('Interface declared both as WebIDL and External interface')
     domInterface = {
         'concrete': False
     }