Add a warning message for matrices that cannot be converted

[binxiangni]

Although just adding the warning message can pass the tests, it will not stop the program and provide a wrong result. Should I replace Rcpp::warning() with Rcpp::stop()?

[eddelbuettel]

Yes, I was thinking that too when I wrote the message over there.

@thirdwing What do you think? stop() or warn() ?

[coatless]

Should be Rcpp::stop().

Prior precedent:

RcppArmadillo/inst/include/RcppArmadilloAs.h

Line 613 in cc04640

Rcpp::stop(msg);