dir/inprocess: give error for Watch of non-existent root
dir/inprocess: give error for Watch of non-existent root

This piece of the specification for Watch was not implemented.

The test will be in the /test directory, similar to the other Watch
tests that exercise this package.

Change-Id: I38eb9f3b7d47ca1eccad22cba4222e238b76e3bb
Reviewed-on: https://upspin-review.googlesource.com/8454
Reviewed-by: Eduardo Pinheiro <c9d6fd76b920b269b838bbb70f683c6db503d8d1@gmail.com>

upspin/upspin,upspin/upspin