fb: Explicitly handle errors when getting or setting a colour palette

In the VESA driver, simply ignore errors. It is not clear to me how to
return them to userspace.

This is in preparation for annotating copyin() and related functions
with __result_use_check.

MFC after: 1 week

(cherry picked from commit ddc8576d297937a1395f47550a8f5b1fac79afc2)