Lay the ground for Rf_error masking (#1402)

[Enchufa2]

First step to #1247

• Add a message at compilation time, no masking yet
• Protect the valid Rf_error calls generated by Rcpp

Checklist

• [x] Code compiles correctly
• [x] R CMD check still passes all tests
• [ ] Preferably, new tests were added which fail without the change
• [x] Document the changes by file in ChangeLog

[Enchufa2]

So, as discussed, and the revdep machine made it clear, we cannot generate a note. Pragma messages are considered statements by gcc (see e.g. here and bugs linked there). Options are: be silent, or generate a warning without replacing.

In order to ask CRAN about what they think, we need to calculate how many packages would generate this warning. An upper bound was provided here by just grepping the sources of all reverse dependencies: 263 packages. From these, only 24 would also fail compilation or checks as documented in #1406. To further refine this list, we could collect the install logs and grep for the pragma (are they in the revdep machine from the previous run?).