Use HTTP 500 in error conditions

Use HTTP 500 in error conditions

Sending a 500 Internal Server Error for unplanned errors is more appropriate
than a 503 Service Unavailable, as the latter is defined:

RFC 7231#6.6.4:

> The 503 (Service Unavailable) status code indicates that the server
> is currently unable to handle the request due to a temporary overload
> or scheduled maintenance, which will likely be alleviated after some
> delay.

It's not likely that the exception will resolve itself after some delay.