mel.rotomap.moles: validate position when loading
mel.rotomap.moles: validate position when loading

Catch errors early by validating input data when loading.

Test Plan:
$ ./meta/run_tests.sh
.. 'mel rotomap-edit' on some moles with floating-point pos, error ..
.. 'mel rotomap-edit' on some moles with regular pos ..

aevri/mel,aevri/mel