expconfig: fix handling of interpolation errors

This is a follow-up of the Python 3 port.
The `message` field is no longer supported in standard exceptions.

expconfig.py

@@ -400,7 +400,7 @@ class ExpConfig(ConfigObj):
                 elif not isinstance(value, dict):
                     value = value.replace('$', '$$')
             except (InterpolationError, ValueError) as error:
-                raise ExpConfigError(error.message, key)
+                raise ExpConfigError(str(error), key)
             section[key] = value
 
         # Undo remaining changes from walk with eval_key
@@ -412,7 +412,7 @@ class ExpConfig(ConfigObj):
                 elif not isinstance(value, dict):
                     value = value.replace('$$', '$')
             except (InterpolationError, ValueError) as error:
-                raise ExpConfigError(error.message, key)
+                raise ExpConfigError(str(error), key)
             section[key] = value
 
         # Move version info from local config to global list