toasterui: proper exit code on toaster errors

toasterui: proper exit code on toaster errors

This patch modifies the toasterui to properly return the exit
code based on the errors found in the toaster itself.

The upload event file API call will not delete event logs for which
toasterui showed an error. This will facilitate debugging.

Minor enhancement in the buildinfohelper to reduce the number
of lookups on unknown layer objects (prevented testing of the patch).

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>