The coding_exception thrown in lib/externallib.php:external_function_info throws exceptions whenever it cannot find any piece of code missing for a defined function. Most of the error messages are too generic so you end up having to add debugging statements into core to find out which function it errored on.

Example:

if (!file_exists($function->classpath)) {

    throw new coding_exception('Can not find file with external function implementation');

}

The coding_exception should all return some data from $function so you can track it down easier.