Issue 2856 asks for a rename of a few methods to a
more idiomatic Go style. This is a very early API
that evolved organically throughout the years.
Together with the fact that ErrorVectors were embedded
in other data structures (e.g. Parser), just renaming
methods (e.g. GetError -> Error) would lead to undesired
behavior (e.g., Parser would act like an Error). Instead,
cleaned up API a bit more:

- removed ErrorVector in favor of ErrorList (already
present)
- simplified Scanner.Init by making the error handler a
function instead of requiring an ErrorHandler implementation
- adjusted helper functions accordingly
- updated Go 1 doc

Fixes #2856.

R=rsc
CC=golang-dev
https://golang.org/cl/5624047