text/template: make redefinition of a template in a set more consistent.

text/template: make redefinition of a template in a set more consistent.
Also make it agree with the documentation. You get an error, unless you're
calling Add explicitly, in which case it panics since that's almost certainly
a bug. The discrepancy was caused by a panic that wasn't turned into
an error along one path; deleted the offending function for clarity.

R=r, rsc
CC=golang-dev
https://golang.org/cl/5354045