WriteHeader may fail to encode a header for any number of reasons,
which can be frustrating for the user when trying to create a tar archive.
As we validate the Header, we generate an informative error message
intended for human consumption and return that if and only if no
format can be selected.

This allows WriteHeader to return informative errors like:
    tar: cannot encode header: invalid PAX record: "linkpath = \x00hello"
    tar: cannot encode header: invalid PAX record: "SCHILY.xattr.foo=bar = baz"
    tar: cannot encode header: Format specifies GNU; and only PAX supports Xattrs
    tar: cannot encode header: Format specifies GNU; and GNU cannot encode ModTime=1969-12-31 15:59:59.0000005 -0800 PST
    tar: cannot encode header: Format specifies GNU; and GNU supports sparse files only with TypeGNUSparse
    tar: cannot encode header: Format specifies USTAR; and USTAR cannot encode ModTime=292277026596-12-04 07:30:07 -0800 PST
    tar: cannot encode header: Format specifies USTAR; and USTAR does not support sparse files
    tar: cannot encode header: Format specifies PAX; and only GNU supports TypeGNUSparse

Updates #18710

Change-Id: I82a498d6f29d02c4e73bce47b768eb578da8499c
Reviewed-on: https://go-review.googlesource.com/58310
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>