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>