compiler: reword "declared and not used" error message

compiler: reword "declared and not used" error message

This is a gofrontend copy of https://golang.org/cl/203282.

From the CL 203282 description:

    "declared and not used" is technically correct, but might confuse
    the user. Switching "and" to "but" will hopefully create the
    contrast for the users: they did one thing (declaration), but
    not the other --- actually using the variable.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/273629