Enable using the N-API C++ wrapper classes with or without C++
exceptions. See the updated `Napi::Error` class documentation for an
overview of the developer experience.

 - Add a `NAPI_CPP_EXCEPTIONS` preprocessor symbol that is defined
   when C++ exceptions are enabled.
 - Add `Env::GetAndClearPendingException()` method.
 - Add `Value::IsEmpty()` method.
 - Update documentation on Error class to provide parallel
   explanation and examples for error-handling without C++ exceptions.
 - Update README to mention optional C++ exception support.
 - Define a `NAPI_THROW_IF_FAILED()` macro that throws either a C++
   or JS exception depending on whether `NAPI_CPP_EXCEPTIONS` is
   defined.
 - Define a `details::WrapCallback()` helper function that catches C++
   exceptions thrown from callbacks, only if `NAPI_CPP_EXCEPTIONS`
   is defined.
 - Update implementation of all methods to use `NAPI_THROW_IF_FAILED()`
   and `details::WrapCallback()` as appropriate.
 - Fix a bug in `Error::New()` when there was a pending exception but
   some different error status was reported by the last error info.
 - Update `test/binding.gyp` to build two separate modules, with and
   without C++ exceptions enabled.
 - Update test JS code to run the same tests against both modules.
 - Update test C++ code to throw JS exceptions (to be compatible with
   both modes).
 - Add some additional test cases to verify expected exceptions are
   observed from JS regardless of whether C++ exceptions are enabled
   or not.
 - Change CI config to ignore failures on nightly builds.