error should be handled nicely

[bangedorrunt]

it's quite annoying when you see an encrypted error like the following, especially for those who are not dev.

Error detected while processing function neuron#edit_zettel_under_cursor[9]..util#handlerr:
line    8:
neuron: E3: no such zettel

the error should be just something like no such zettel, zettel is not found ...