Fix s3 fips endpoint

There were two issues that led to this bug.

1.  The _regions.json defined fips as fips-us-gov-west-1 but the s3.extra.json defined this region as fips-gov-west-1.
2. `get_endpoint()` would return the global endpoint if one was defined whenever an unknown region_name was given (`botocore.session.get_session().get_service('s3').get_endpoint('DOES NOT EXIST')`)

As a result, the special casing for the fips govcloud region was not being handled correctly.  In order to address this I've changed the region name in s3.extra.json to match what's in _regions.json.  I've also added a test specifically for fips.

I also started to fix the get_endpoint() method by raising an exception when you try to create an endpoint for an known region, but for the time being I have to leave it in.  The added test discusses this behavior, and it's something we might be able to change later, but for now I think we have to leave it in.

cc @danielgtaylor