gracefully trap incorrect returnType in `nCompile`

[paciorek]

We currently get C++ compilation errors when the type of return does not match returnType. E.g.,

foo = nFunction(
              fun = function() {
                  y <- rnorm(4)
                  return(y)
              }, returnType = 'integerVector')
cfoo <- nCompile(foo)

I am not printing the error message on account of length, but it occurs with the Eigen code.

This also occurs in a way that will particularly confuse users in cases where they will probably expect integer type:

foo = nFunction(
              fun = function() {
                  y <- 1:3
                  return(y)
              }, returnType = 'numericVector')
cfoo <- nCompile(foo)

The above is fine, but if returnType = 'integerVector' (as an R user would generally do), then one gets the C++ compilation error.

[perrydv]

I started working on this in the branch return-casting, but (as implied by the name) also got distracted by not just error trapping but also improving what it does.