`source` throws an exception if keyboard cursor pos is invalid

[ab9rf]

source uses the keyboard cursor for its location determination but doesn't handle errors well

per discord

[Alistair-Afton]

@ab9rf Fixed in DFHack/scripts#1629. The cursor is now validated before the source lookup, and both add and delete have regression coverage.

[ab9rf]

for the record, it's not necessary to ping me on issues i opened. as one of the organization's managers, i'm notified for all activity anywhere in the organization :)