vboot/ui: Refactor ui error handling

Add helper functions to check and set ui error code. The ui error
handling shouldn't catch the requests but only the error.

Add vb2_is_error() to 2api.h. This function is for checking if the
return value is an error or not.

TEST=make clean && CC=x86_64-pc-linux-gnu-clang make runtests

Signed-off-by: Chung-Sheng Wu <chungsheng@google.com>
Change-Id: I5c9a34dadf749f3b5364860a1a034bfefe0a61f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2780821
Tested-by: Chung-Sheng Wu <chungsheng@chromium.org>
Commit-Queue: Chung-Sheng Wu <chungsheng@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-by: Joel Kitching <kitching@chromium.org>
3 files changed