go/parser: slightly improved error message by adding hint
go/parser: slightly improved error message by adding hint

It's difficult to make this much better w/o much
more effort. This is a rare case and probably not
worth it.

Fixes issue 6052.

R=golang-codereviews, bradfitz, adonovan
CC=golang-codereviews
https://codereview.appspot.com/49740045

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang