Stack trace for allow.cartesian error is ugly

[MichaelChirico]

Error in vecseq(f__, len__, if (allow.cartesian || notjoin || !anyDuplicated(f__,  : 
  Join results in 6 rows; more than 5 = nrow(x)+nrow(i). Check for duplicate key values in i each of which join to the same group in x over and over again. If that's ok, try by=.EACHI to run j for each group to avoid the large allocation. If you are sure you wish to proceed, rerun with allow.cartesian=TRUE. Otherwise, please search for this error message in the FAQ, Wiki, Stack Overflow and data.table issue tracker for advice.

Error in vecseq(f__, len__, if (allow.cartesian || notjoin || !anyDuplicated(f__, :

user doesn't need to see that / it adds a lot of distracting noise to the info we present the user.

I'm not aware of a way to set call.=FALSE for errors generated in C with error(), so instead I propose capturing that if() condition in a variable to at least make the error call simpler.

[MichaelChirico]

Closed already in #6436 (split off from #4370)

[aitap]

a way to set call.=FALSE for errors generated in C with error()

Rf_errorcall(R_NilValue, format, ...) should help in similar cases.