Remove jobs from assigned_jobs when allocation fails.
Remove jobs from assigned_jobs when allocation fails.

In practice servers will occasionally become unreachable and trying
to allocate to them will fail. Rather than leaking capacity in this
case we remove the job on allocation failure. Fixes #477

glandium/sccache,luser/sccache2,glandium/sccache,luser/sccache2,mozilla/sccache,luser/sccache2,mozilla/sccache,mozilla/sccache,glandium/sccache,luser/sccache2,glandium/sccache