errors: use runtime.CallersFrames to walk stack
errors: use runtime.CallersFrames to walk stack

With inlining, going direct from Callers to Funcs is error-prone in
the new Go release.

Fixes #223.

Change-Id: If10a275fe793d1da0ff7800aced7078c23f60d90
Reviewed-on: https://upspin-review.googlesource.com/8262
Reviewed-by: Eduardo Pinheiro <c9d6fd76b920b269b838bbb70f683c6db503d8d1@gmail.com>

upspin/upspin,upspin/upspin