cmd/trace: fix a javascript bug in handling import error

When traceviewer encounters a failure of json trace import
due to data error, onImportFail tried to access an error variable
which was not yet defined.

Change-Id: I431be03f179aafacaf1fd3c62a6337e8b5bd18fb
Reviewed-on: https://go-review.googlesource.com/71970Reviewed-by: Austin Clements <austin@google.com>

src/cmd/trace/trace.go

@@ -114,7 +114,7 @@ var templTrace = `
     viewer.viewTitle = "trace";
   }
 
-  function onImportFail() {
+  function onImportFail(err) {
     var overlay = new tr.ui.b.Overlay();
     overlay.textContent = tr.b.normalizeException(err).message;
     overlay.title = 'Import error';