Calling `rapi_error_with_context()` is unsafe from ALTREP methods

`rapi_error_with_context()` jumps into R, this appears to be unsafe or unsupported for ALTREP methods. I have seen stack overflows occur from this.

Define a guard that is activated whenever an ALTREP method is entered. Query this guard in `rapi_error_with_context()` and fall back to throwing an exception instead of calling into R via `rapi_error()` .