add informative error message to facilitate debugging

[jasmineirx]

I was running a simulation and encountered an uninformative error message. I was able to track down the source but it took me a while since the error was very generic:

Error in vapply(regimen$type, function(x) cmt_mapping[[x]], FUN.VALUE = numeric(1), :
values must be length 1,
but FUN(X[[1]]) result is length 0

Having this extra check and more informative error message would have saved me some time.

[dominic-irx]

lgtm, tricky!

[roninsightrx]

👍🏻