testing: Make handlers.Error obey the standard 'error' interface

And return these types from test helper functions

internal/handlers/errors.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"encoding/json"
+	"fmt"
 	"log"
 	"net/http"
 	"strings"
@@ -12,6 +13,10 @@ type Error struct {
 	ErrorString string
 }
 
+func (e *Error) Error() string {
+	return fmt.Sprintf("Error %d: %s", e.ErrorId, e.ErrorString)
+}
+
 func (e *Error) Read(json_str string) error {
 	dec := json.NewDecoder(strings.NewReader(json_str))
 	return dec.Decode(e)