[5.x] Throw better exception when asset isn't found (#11321)

Author: edalzell

src/Fieldtypes/Assets/Assets.php

@@ -161,7 +161,7 @@ public function process($data)
         $max_files = (int) $this->config('max_files');
 
         $values = collect($data)->map(function ($id) {
-            return Asset::find($id)->path();
+            return Asset::findOrFail($id)->path();
         });
 
         return $this->config('max_files') === 1 ? $values->first() : $values->all();