cmd/compile/internal/syntax: follow Go naming conventions for error methods

Also, remove parser.error method (in favor of parser.errorAt) as it's only
used twice.

This is a purely cosmetic change.

Change-Id: Idb3b8b50f1c2e4d10de2ffb1c1184ceba8f7de8a
Reviewed-on: https://go-review.googlesource.com/94030Reviewed-by: Matthew Dempsky <mdempsky@google.com>