don't rely on exception for http 416

don't rely on exception for http 416

Fix a bug that could make resuming a download from the web fail when the
entire content of the file is actually already present locally.

What a mess that Request can throw exceptions or not, depending on how
it's configured. Makes it very hard if you need to handle some specific
http status codes in a function like this! Implementing everything two
ways did not seem appealing, if possible at all, so I decided to
override the Request if it did come configured to throw exception on
non-2xx http status. Other exceptions, like from http-client-restricted,
or due to a redirect to a non-http url, still get thrown.

This commit was sponsored by Luke Shumaker on Patreon.