Cast UploadWizardErrorFlowEvent.message to string

Sometimes this parameter is a function, which causes the
EventLogging type check to error out. This way the message
will become '[object Function]' which is not helpful but at
least does not break the logging.

Change-Id: I093769d34d82c52dfc31804b03998b240e1d3619
Merge "Cast UploadWizardErrorFlowEvent.message to string"