BAD_REQUEST should not be changed to ReplyError

[sssoleileraaa]

Description

The SDK can make it difficult to debug if we change the types of errors that we get back from the server, see https://github.com/freedomofpress/securedrop-client/pull/1165#pullrequestreview-529168096, for example, where BAD_REQUEST for a seen request is returned to the client as a ReplyError: https://github.com/freedomofpress/securedrop-sdk/blob/cc5eea0dd4db75f4524ab3d99a31f8efb5a90139/sdclientapi/init.py#L205-L206. The client does a lot of response error handling, such as retrying a request based on request type and error, so we should make sure there are not other places in the SDK code that change error types.