[webui] Fix 500 error when looking for a build log of a non existant project

We had 2 uncatched exceptions in the live_build_log action: Project::UnknownObjectError and
Package::UnknownObjectError. With this commit we will catch those errors and show a flash
error instead.

Related errbit error:
  https://errbit-opensuse.herokuapp.com/apps/5620ca2bdc71fa00b1000000/problems/564ee010ae70420009000004