job-manager: improve error message from limit-duration plugin

Problem: The limit-duration plugin does not include the requested
duration or target queue in the error message when rejecting a job.
This can lead to user confusion about the source of the policy limit.

Add the requested duration (formatted as fsd) and the queue (if any)
to the error message sent back to the user.

Author: grondo

src/modules/job-manager/plugins/limit-duration.c

@@ -193,17 +193,24 @@ static int check_limit (struct limit_duration *ctx,
 {
     double limit = ctx->general_limit;
     double qlimit = queues_lookup (ctx->queues, queue);
+    bool unlimited = duration == DURATION_UNLIMITED;
 
     if (qlimit != DURATION_INVALID)
         limit = qlimit;
     if (limit != DURATION_INVALID
         && limit != DURATION_UNLIMITED
-        && (duration > limit || duration == DURATION_UNLIMITED)) {
+        && (duration > limit || unlimited)) {
+        char requested[64];
         char fsd[64];
+        fsd_format_duration_ex (requested, sizeof (requested), duration, 2);
         fsd_format_duration_ex (fsd, sizeof (fsd), limit, 2);
         return errprintf (error,
-                          "requested duration exceeds policy limit of %s",
-                          fsd);
+                          "requested duration (%s) exceeds policy limit of "
+                          "%s%s%s",
+                          unlimited ? "unlimited" : requested,
+                          fsd,
+                          queue ? " for queue " : "",
+                          queue ? queue : "");
     }
     return 0;
 }