Replace warning by error message for case in which: missing 1 time stamps at the beginning

[treerink]

In some cases flux values are missing at the start of an experiment, in order to provide a full time record in that case for this initial time slice a field with missing values is added.

However, in case where something else goes wrong (for instance #725) this can also occur:

2021-07-17 20:48:41 WARNING:ece2cmor3.ifs2cmor: For variable ua in table 6hrLev, no valid time point could be found at 1950-01-01 00:00:00...inserting missing values
2021-07-17 20:49:09 WARNING:ece2cmor3.ifs2cmor: The file /lustre3/projects/CMIP6/reerink/temp-cmor-dir/cmip6-cordex-model-levels/h003/ifs/101/h003-ifs-1950/ta_6hrLev.nc seems to be missing 1 time stamps at the beginning, these will be added

Therefore it might be better to give an ERROR:ece2cmor3.ifs2cmor message instead of the warning, so it will be easier detected. Eventually with a possible error situation message. Because once the cmorised data has the missing values, nctime is not detecting as erroneous.