PLT-2952 fixing preview mode error message (#3113)

webapp/stores/error_store.jsx

@@ -64,8 +64,10 @@ class ErrorStoreClass extends EventEmitter {
 
     clearLastError() {
         var lastError = this.getLastError();
+
+        // preview message can only be cleared by clearPreviewError
         if (lastError && lastError.email_preview) {
-            this.ignore_email_preview = true;
+            return;
         }
 
         BrowserStore.removeGlobalItem('last_error');
@@ -74,6 +76,12 @@ class ErrorStoreClass extends EventEmitter {
             this.emitChange();
         }
     }
+
+    clearPreviewError() {
+        this.ignore_email_preview = true;
+        this.storeLastError('');
+        this.clearLastError();
+    }
 }
 
 var ErrorStore = new ErrorStoreClass();