api/txpool/broadcast HTTP status error handling

[Alrighttt]

Current Behavior

Using the endpoint, POST api/txpool/broadcast, if an invalid transaction is provided, the user will receive a HTTP 500 response.

Example

invalid v2 transaction set: transaction txid:04ad8fd61964efc37f8df21a43a3f792e03635e0efc604d52aff58c53d35657e is invalid: siacoin input 0 failed to satisfy spend policy: invalid signature

Expected Behavior

I would expect to receive a HTTP status code of 400 since this is an error in the client behavior, providing an invalid signature.

Steps to Reproduce

1. Attempt to send a transaction via POST api/txpool/broadcast with an invalid signature

Version

b256f1a

What operating system did the problem occur on (e.g. Ubuntu 22.04, macOS 12.0, Windows 11)?

docker image built from Dockerfile / alpine

Anything else?

I've not looked in depth for any other instances of this, but it's worth looking at other endpoints to see if any client related errors also return HTTP 500.