Standardize error response in three request handlers

[nichamon]

Modify updtr_task, prdcr_hint, and set_route handlers to use ldmsd_send_req_response() for sending error messages back to requesters. This function automatically handles message length calculation, removing the need for manual length calculations in these handlers.

[nichamon]

This change is based on code inspection rather than an observed issue. It standardizes the error response handling in these handlers by using ldmsd_send_req_response() for consistent message length calculation.

[tom95858]

@nichamon does anyone still use ldmsd_send_error_reply?

[nichamon]

@nichamon does anyone still use ldmsd_send_error_reply?

Yes, ldmsd_process_config_request() calls ldmsd_send_error_reply() to send an error when an error occurs while preparing the request context ldmsd_req_ctxt reqc.

[tom95858]

@nichamon, I'm a little bit concerned that this code is essentially untested unless we force the error path. Have we tried to do that?

[nichamon]

@nichamon, I'm a little bit concerned that this code is essentially untested unless we force the error path. Have we tried to do that?

@tom95858 Yes, I tested by instrumenting the code to and force it to go through the error paths. While I tested the change in the set_route path, I found bugs in the python code that handles the set_route command. The fix is in PR #1552.