MDL-34065 lib: improve two debugging messages.

If the string passed to get_string is empty, say that. Don't say that it
contains illegal characters.

When relying on the __call magic in plugin_renderer_base, when the
method cannot be found, include the right class name in the error
message.

lib/moodlelib.php

@@ -7179,7 +7179,7 @@ function get_string($identifier, $component = '', $a = NULL, $lazyload = false)
 
     $identifier = clean_param($identifier, PARAM_STRINGID);
     if (empty($identifier)) {
-        throw new coding_exception('Invalid string identifier. Most probably some illegal character is part of the string identifier. Please fix your get_string() call and string definition');
+        throw new coding_exception('Invalid string identifier. The identifier cannot be empty. Please fix your get_string() call.');
     }
 
     // There is now a forth argument again, this time it is a boolean however so