bsdinstall: Handle errors from geom_gettree.

geom_gettree probably never fails, and if it does there isn't much of
a fallback other than aborting partitioning. However, a few places
were checking the return value and not doing anything with it
triggering a unused-but-set-variable warning. Checking the errors
resolves the warning.

While here, check for errors in other places that weren't checking for
them at all, remove a spurious double call (the second call overwrote
the mesh structure leaking all the pointers from the first), and close
a few resource leaks on error paths.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D40779