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

lib/outputrenderers.php

@@ -228,12 +228,12 @@ class plugin_renderer_base extends renderer_base {
      */
     public function __call($method, $arguments) {
         if (method_exists('renderer_base', $method)) {
-            throw new coding_exception('Protected method called against '.__CLASS__.' :: '.$method);
+            throw new coding_exception('Protected method called against '.get_class($this).' :: '.$method);
         }
         if (method_exists($this->output, $method)) {
             return call_user_func_array(array($this->output, $method), $arguments);
         } else {
-            throw new coding_exception('Unknown method called against '.__CLASS__.' :: '.$method);
+            throw new coding_exception('Unknown method called against '.get_class($this).' :: '.$method);
         }
     }
 }