Add error propagation to web UI via special JSON error type.

rules/ast/printer.go

@@ -132,10 +132,10 @@ func (matrix Matrix) ToString() string {
 func ErrorToJSON(err error) string {
 	errorStruct := struct {
 		Type  string
-		Error string
+		Value string
 	}{
 		Type:  "error",
-		Error: err.Error(),
+		Value: err.Error(),
 	}
 
 	errorJSON, err := json.MarshalIndent(errorStruct, "", "\t")

static/js/rickshaw.js

@@ -84,6 +84,10 @@ function submitQuery() {
       dataType: "json",
       data: form.serialize(),
       success: function(json, textStatus) {
+        if (json.Type == "error") {
+          alert(json.Value);
+          return;
+        }
         data = transformData(json);
         if (data.length == 0) {
           alert("No datapoints found.");