Improve "Cannot find file with external function implementation" exception

While creating a new web service and adding new functions to it, I got the following error/exception "Cannot find file with external function implementation" due to erroneous "externallib.php" file from one of the contrib plugins installed on my system.

The exception does not show which plugin was causing this error, and so I added the full reference to the problematic missing file to the output of the exception.

It seems easy and trivial to add this improvement to the code, especially for cases where the system admin can see what is causing the error and remove/disable the problematic plugin without the immediate need for a developer. (Obviously, it should ultimately be solved by a developer with access to the code)

Suggested patch:

// https://github.com/moodle/moodle/blob/master/lib/externallib.php#L87

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

  throw new coding_exception('Cannot find file with external function implementation = '.$function->classpath);

}