Revert quitting to warning for missing/misspelled antentenna in AntennaConstrain
With DP3 v>5.1 a missing antenna listed in AntennaConstrain makes DP3 quit. This makes pipelines that has to calibrate thousands of datasets more complicated as they need to check every time which antenna is present in an observation and construct the parset accordingly. A "Warning", without a quit (as it was in DP3 v5.1), would solve the problem.