resolve #58 return error response

alerting/src/main/kotlin/com/amazon/opendistroforelasticsearch/alerting/model/MonitorRunResult.kt

@@ -77,10 +77,14 @@ data class TriggerRunResult(
     val actionResults: MutableMap<String, ActionRunResult> = mutableMapOf()
 ) : ToXContent {
     override fun toXContent(builder: XContentBuilder, params: ToXContent.Params): XContentBuilder {
+        var msg = error?.message;
+        if(error is ScriptException){
+            msg = error.toJsonString()
+        }     
         return builder.startObject()
                 .field("name", triggerName)
                 .field("triggered", triggered)
-                .field("error", error?.message)
+                .field("error", msg)
                 .field("action_results", actionResults as Map<String, ActionRunResult>)
                 .endObject()
     }