All errors are handled as 500
For example: 
• Request /v1/pipelines/{{pipeline_id}}/start is returned code 500 after trying start the pipeline which don't have connectors or have only 1 connector
message: "failed to start pipeline: could not build nodes for pipeline {id} can't build pipeline without any source connectors"
• Request /v1/connectors is returned code 500 after trying create connector with an invalid configuration
"message": "failed to create connector: could not init connector: invalid plugin config: operation error S3: HeadBucket, serialization failed: serialization failed: input member Bucket must not be empty"

Maybe it could be a 400 code?
And these error messages are not documented in Swagger.