Fix printing of errors for unknown codes.

pyopencl/__init__.py

@@ -333,7 +333,8 @@ def _add_functionality():
     def error_str(self):
         val = self.args[0]
         result = "%s failed: %s" % (val.routine(), 
-                status_code.to_string(val.code()).lower().replace("_", " "))
+                status_code.to_string(val.code(), "<unknown error %d>")
+                .lower().replace("_", " "))
         if val.what():
             result += " - " + val.what()
         return result