libavb: Handle wrong hashtree_error_mode in avb_append_options()

Exit with AVB_SLOT_VERIFY_RESULT_ERROR_INVALID_ARGUMENT
when hashtree_error_mode value passed to avb_append_options()
is unknown (not from AvbHashtreeErrorMode enum).

Otherwise, default value is not handled in the
switch(hashtree_error_mode), which causes below compile warning:

lib/libavb/avb_cmdline.c: In function ‘avb_append_options’:
lib/libavb/avb_cmdline.c:354:13: warning: ‘dm_verity_mode’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     new_ret = avb_replace(
     ~~~~~~~~^~~~~~~~~~~~~~
         slot_data->cmdline, "$(ANDROID_VERITY_MODE)", dm_verity_mode);
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lib/libavb/avb_cmdline.c:363:8: warning: ‘verity_mode’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   if (!cmdline_append_option(
        ^~~~~~~~~~~~~~~~~~~~~~
           slot_data, "androidboot.veritymode", verity_mode)) {

Signed-off-by: Ievgen Maliarenko <ievgen.maliarenko@globallogic.com>
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>