text/template: improve nil errors in evalField

If we're accessing a field on a nil struct pointer, and that field is
present in the type, we should print a "nil pointer evaluating X.Y" error
instead of the broader "can't evaluate field Y in X". The latter error
should still be used for the cases where the field is simply missing.

While at it, remove the isNil checks in the struct and map cases. The
indirect func will only return a true isNil when returning a pointer or
interface reflect.Value, so it's impossible for either of these checks
to be useful.

Finally, extend the test suite to test a handful of these edge cases,
including the one shown in the original issue.

Fixes #29137.

Change-Id: I53408ced8a7b53807a0a8461b6baef1cd01d25ae
Reviewed-on: https://go-review.googlesource.com/c/153341


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>