BROWSE: Return R_UNSET rather than R_NONE.
BROWSE: Return R_UNSET rather than R_NONE.

Following discussion on AltMe, modify BROWSE to return R_UNSET rather
than R_NONE.

In addition, handle the NONE case correctly (return early).

Pointillistic/rebol-lang,zsx/r3,zsx/r3,Pointillistic/rebol-lang,zsx/r3,zsx/r3,Pointillistic/rebol-lang,Pointillistic/rebol-lang