dir/inprocess: use errors.Is instead of errors.Match
dir/inprocess: use errors.Is instead of errors.Match

Change-Id: If65c7793f08075001f990f092987df098b5e82cf
Reviewed-on: https://upspin-review.googlesource.com/16940
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin