Add check for Git CondDB tag metadata before publishing an update

Recently it happened (again) that a typo in the Git CondDB tag message caused troubles in the nightly builds (nothing reported from production). See lhcb-conddb/SIMCOND#8 (comment 3954283)

We should make sure that all tag messages are valid YAML documents as by https://gitlab.cern.ch/lhcb/LHCb/-/blob/master/Tools/GitEntityResolver/python/GitCondDB/Tags.py#L24