software/theia: fix resiliency test after upgrade to slapos.core 1.12.0

After the change in https://lab.nexedi.com/nexedi/slapos.core/-/blob/2b80692d62b3dbd9e67d70d6143f180fc2f155fc/slapos/util.py#L449-453, there is now a KeyError message if the serialisation is not found. The theia test was actively checking there is no "Error" in the output log hence it was now failing.

Allow just this specific error.