BUG 8917 No error message displayed if a case executes an external... SOLVER

- In the case step was not performed for external verification step, the case if a plugin existed or not.
- Added a verification whether or not there is a plugin for an external step, plus add a label in os environment.

workflow/engine/methods/cases/cases_Step.php

@@ -986,8 +986,10 @@
         if ( $val->sStepId == $_GET['UID'] ) {
           $sNamespace = $val->sNamespace;
           $sStepName  = $val->sStepName;
+          
         }
       }
+      if ( class_exists($sNamespace."plugin")) {
         if (!$aPreviousStep) {
           $Fields['APP_DATA']['__DYNAFORM_OPTIONS']['PREVIOUS_STEP_LABEL'] = '';
         }
@@ -1010,8 +1012,12 @@
         }");
   
         $G_PUBLISH->AddContent('content', $stepFilename);
+      }
+      else {
+        $aMessage['MESSAGE'] = G::loadTranslation('ID_EXTERNAL_STEP_MISSING', SYS_LANG, array("plugin"=>$sNamespace) );
+        $G_PUBLISH->AddContent('xmlform', 'xmlform', 'login/showMessage', '', $aMessage );
+      }
       break;
-
     }
     //Add content content step - End
   }