internal/pkgbits: s/errorf/panicf/ because that's what it is

Make it obvious that this function panics.

Change-Id: I272142d2cf7132aa8915f8f4b5945834376db062
Reviewed-on: https://go-review.googlesource.com/c/go/+/606935


Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>