[aggr-] catch nonexistent aggr in addcol-aggregate

[midichef]

A fix to add error checking for addcol-aggregate. If I run addcol-aggregate, then enter any string that is not an aggregator, like noop, I get the appropriate warning: aggregator does not exist: noop but I also get a traceback:

File "/home/midichef/.venv/lib/python3.12/site-packages/visidata/aggregators.py", line 226, in aggregate_groups
    aggr_val = aggr.aggregate(col, [rows[rownum] for _, _, rownum in group])
AttributeError: 'NoneType' object has no attribute 'aggregate'