go/scanner: minimal non-terminated literals

go/scanner: minimal non-terminated literals

Consume as little as possible input when encountering
non-terminated rune, string, and raw string literals.
The old code consumed at least one extra character
which could lead to worse error recovery when parsing
erroneous sources.

Also made error messages in those cases more consistent.

Fixes #7091.

R=adonovan
CC=golang-codereviews
https://golang.org/cl/50630043