Pass fastapi.HTTPExceptions through truss server.

[marius-baseten]

🚀 What

Add branch to truss server error handling that passes through fastapi.HTTPExceptions if raised in model code.

This allows to propagate more specific error messages/codes/details to the client.

💻 How

🔬 Testing

[squidarth]

thanks for writing the integration test here! I think this is fine, but I do want to flag what we discussed earlier (we shouldn't advertise this super widely so we have the flexibility to move off of fastapi if we need to)