scanner: error handler must be provided to Init

scanner: error handler must be provided to Init

Init may report an error on the first character and
thus one needs an ability to set the error handler
for Init. Was a design bug.

Added corresponding test cases and better documentation.
Also: Fixed a subtle infinite loop exposed by one of the
new test cases.

Fixes #1380.

R=rsc, gri
CC=golang-dev
https://golang.org/cl/4094041