Most 400 series errors are caused by user error. In most cases, the system will return a user friendly message for user errors. For example, if I try to create a work order without the required data:
Client applications should always try and deserialize this information for display to a user or for logging.
In some cases the error will not follow this format. For example, performing a post without Content-Type specified will return this rather unfriendly error:
This type of error is generally only returned due to developer error (not user data entry error).