Move all sentinel errors to common.go since some of them are
returned by both the reader and writer and remove errInvalidHeader
since it not used.

Also, consistently use the "tar: " prefix for errors.

Change-Id: I0afb185bbf3db80dfd9595321603924454a4c2f9
Reviewed-on: https://go-review.googlesource.com/55650Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>