Output a more useful error message if you don't register a function

[jimhester]

Right now you get

Error: Can't subset columns that don't exist.
x Columns `name`, `return_type`, and `args` don't exist.

Which is not very helpful.

[sbearrows]

I tried recreating the error using devtools::load_all() in an R package by removing cpp11 decorators but I was still only able to recreate it when the content between [[]] was not cpp11::register (ex [[cpp11:register]], [[cpp11::egister]], etc). If there are no decorators it doesn't give any error. If there is at least one other function with the correct decorator (so one with the correct decorator and one with the incorrect decorator) the compiler gives an error that is fairly informative. I'll move ahead with the solution we discussed earlier!