Allow trycatch to call Fail using std::string

[dtolnay]

Before:

template <typename Try, typename Fail>
static void trycatch(Try &&func, Fail &&fail) noexcept try {
  func();
} catch (const whatever &ex) {
  std::ostringstream buf;
  ex.dump(buf);
  fail(buf.str().c_str());
}

After: just fail(buf.str()) without .c_str(), and performs better because we bypass a redundant strlen computation.