debug/dwarf: return ErrUnknownPC rather than nil on unknown PC
Currently, on e == nil or e.Tag == 0, SeekPC returns with a nil error.
Instead, indicate that the PC is unknown.
Change-Id: I9594296034e2df872e399bd800b00cb565c413c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/473695 Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>