Inexplicable error message when probeTemplate is missing in MultiRow

[agmenc]

Change the header row cells from td to th. This causes an error message.

Firstly, this should be handled (it is perfectly valid).

Secondly, the error message is just "can't call tail on an empty list".

If we get to MultiRow.probeForCell and there is no probe template, abort that table with a helpful error message.