errors: record only single frame

See Issue #29382 and Issue #30468.

3 frames are no longer needed as of
https://go-review.googlesource.com/c/go/+/152537/

name                     old time/op  new time/op  delta
New-8                     475ns ± 3%   352ns ± 2%  -25.87%  (p=0.008 n=5+5)
Errorf/no_format-8        661ns ± 4%   558ns ± 2%  -15.63%  (p=0.008 n=5+5)
Errorf/with_format-8      729ns ± 6%   626ns ± 2%  -14.23%  (p=0.008 n=5+5)
Errorf/method:_mytype-8  1.00µs ± 9%  0.84µs ± 2%  -15.94%  (p=0.008 n=5+5)
Errorf/method:_number-8  1.25µs ± 7%  1.04µs ± 2%  -16.38%  (p=0.008 n=5+5)

Change-Id: I30377e769b3b3be623f63ecbe365f8950ca08dda
Reviewed-on: https://go-review.googlesource.com/c/go/+/167400


Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>