For example: Apache (httpd) replaces the 4xx and 5xx response body with its own content instead of whatever you'd returned from an external handler like wsgi. You have to use a 2xx (except for 204) to get a relevant error message back out.
This is the default behavior. Apache httpd can be configured to produce different responses by way of ErrorDocument[0]. From the documentation:
Customized error responses can be defined for any HTTP
status code designated as an error condition - that is,
any 4xx or 5xx status.