[web] Return status when returning headers

Return status as headers['_http_status'] if return_headers is set to True.
I'm using '_http_status' so it won't override any actual HTTP header that might
be present in the response.

This commit might break your modules if you didn't treat the returned headers
as a plain dict. I doubt anyone actually does that anyway.
[currency] Better error message for invalid currencies

Fixes #552