Fix some _doebuild_exit_status_check() calls so that they only happen when

Fix some _doebuild_exit_status_check() calls so that they only happen when
the bash process exits successfully (for consistency with usage elsewhere).
When calling this functions, we're mainly concerned about false success, and
we always want to allow things like using bashrc die hooks to clean up
$PORTAGE_BUILDDIR for users that are building in tmpfs (in which case the
exit status file may be removed before bash exits).

svn path=/main/trunk/; revision=13296