app/build/handler: error out telling which version we got

Change-Id: I789d420279fbc1c27fec02ef4c7556b2f9997b1c
Reviewed-on: https://go-review.googlesource.com/17290
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/app/build/handler.go b/app/build/handler.go
index d2ae8b2..426d768 100644
--- a/app/build/handler.go
+++ b/app/build/handler.go
@@ -95,7 +95,8 @@
 	if dashboardForRequest(r) != gccgoDash {
 		v, _ := strconv.Atoi(r.FormValue("version"))
 		if v != watcherVersion {
-			return nil, fmt.Errorf("rejecting POST from commit watcher; need version %v", watcherVersion)
+			return nil, fmt.Errorf("rejecting POST from commit watcher; need version %v instead of %v",
+				watcherVersion, v)
 		}
 	}
 
@@ -579,7 +580,8 @@
 	if dashboardForRequest(r) != gccgoDash {
 		v, _ := strconv.Atoi(r.FormValue("version"))
 		if v != builderVersion {
-			return nil, fmt.Errorf("rejecting POST from builder; need version %v", builderVersion)
+			return nil, fmt.Errorf("rejecting POST from builder; need version %v instead of %v",
+				builderVersion, v)
 		}
 	}