They now show the correct name, the byte offset on the line, and context for the failed evaluation.
Before:
        template: three:7: error calling index: index out of range: 5
After:
        template: top:7:20: executing "three" at <index "hi" $>: error calling index: index out of range: 5
Here top is the template that was parsed to create the set, and the error appears with the action
starting at byte 20 of line 7 of "top", inside the template called "three", evaluating the expression
<index "hi" $>.

Also fix a bug in index: it didn't work on strings. Ouch.

Also fix bug in error for index: was showing type of index not slice.
The real previous error was:
        template: three:7: error calling index: can't index item of type int
The html/template package's errors can be improved by building on this;
I'll do that in a separate pass.

Extends the API for text/template/parse but only by addition of a field and method. The
old API still works.

Fixes #3188.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6576058