santiago.roche@mongodb.com found an inconsistency with extension error-handling:

Basically, the problem is that we report errors from get_mongodb_extension() via a MongoExtensionStatus*. However, the only way to truly throw a server style uassert/tassert is via the HostServices*, which we only set on an extension, once an extension is returned to the server...

I think we might want to consider modifying the get_mongodb_extension function signature to accept a vector of HostServices* (one for each API version the host supports). Then we could set the host services pointer during the get_mongodb_extension  function call. 

We should investigate the inconsistency with extension loading and fix it so that all extension errors can be generated from HostServices.