Fix run now errors and interrupts

[jcheng5]

Fixes #191

[jcheng5]

Re: remaining failures for R 4.1 and 3.6 on Windows: On these builds, having Rcpp code throw an exception that's caught by the calling R code, seems to be escaping the try/catch somehow. But only for i386, not x64!?

[jcheng5]

Minimal repro looks like the below. It actually works when called from RStudio, but fails when called from R at a terminal or if you put it in a file and call R -f test.R.

library(later)

Rcpp::cppFunction(
  depends = "later",
  includes = '
      #include <later_api.h>

      void oof(void* data) {
        throw std::runtime_error("This is a C++ exception.");
      }
    ',
  code = '
      void cpp_error(int value) {
        later::later(oof, 0, 0);
      }
    '
)

tryCatch(
  {cpp_error(1); later::run_now()},
  error = function(e) {message("got here")}
)