Update Error Message for Custom Function Invocation

[xterao]

Current Behavior

• The class providing custom-function definitions is specified via doma.compile.config.
• A cache is refreshed only when that file’s last-modified timestamp changes.

Problem

If a custom function call is mistakenly highlighted as an error before the cache refreshes, the existing error message is unclear.

Required Fix

Replace the current message with one that explicitly tells the user to re-save doma.compile.config so the cache can update and clear the false error.