gofmt: if a semicolon is found unexpectedly, report detailed cause

gofmt: if a semicolon is found unexpectedly, report detailed cause

go/scanner: return information on semicolon (real or inserted) when
	    found in source
go/parser:  better error message when a semicolon is found unexpectedly

For instance, if an unexpected semicolon is found that was automatically
inserted, the parser error message is now:

    "expected '}', found newline"

Fixes #1006.

R=rsc
CC=golang-dev
https://golang.org/cl/1936044