BUGFIX: __toString must not throw an excaption

When an image is not found due to security constraints, an exception is thrown
which is covered by PHPs exception that __toString should never throw an exception.

Classes/EelHelpers/AbstractImageSourceHelper.php

@@ -118,14 +118,17 @@ public function allowsCallOfMethod($methodName)
         }
     }
 
-
     /**
      * If the source is cast to string the default source is returned
      *
      * @return string
      */
     public function __toString(): string
     {
-        return $this->src();
+        try {
+            return $this->src();
+        } catch (\Exception $e) {
+            return $e->getMessage();
+        }
     }
 }