exp/template/html: moved error docs out of package docs onto error codes

exp/template/html: moved error docs out of package docs onto error codes

This replaces the errStr & errLine members of context with a single err
*Error, and introduces a number of const error codes, one per
escape-time failure mode, that can be separately documented.

The changes to the error documentation moved from doc.go to error.go
are cosmetic.

R=r, nigeltao
CC=golang-dev
https://golang.org/cl/5026041