fix(napi): improve error propagation

[devongovett]

This enables napi::Error to wrap an arbitrary JavaScript value, which may have been returned or thrown from a function. This is useful for functions that must return an napi::Result, but may have an error propagated from calling a JavaScript function. The support was partially already there for the Promise implementation, but I've extended it.

This also checks if an exception is pending after calling a JS function, and retrieves it with napi_get_and_clear_last_exception. This way, the pending exception isn't necessarily immediately thrown when returning from the native function, but can be passed elsewhere if needed (e.g. to an error callback). This is also important when calling JS functions outside the native call context (e.g. from a background thread).