blockjob: report a better error message

When a block job fails, we report strerror(-job->job.ret) error
message, also if the job set an error object.
Let's report a better error message using error_get_pretty(job->job.err).

If an error object was not set, strerror(-job->ret) is used as fallback,
as explained in include/qemu/job.h:

typedef struct Job {
    ...
    /**
     * Error object for a failed job.
     * If job->ret is nonzero and an error object was not set, it will be set
     * to strerror(-job->ret) during job_completed.
     */
    Error *err;
}

In block_job_query() there can be a transient where 'job.err' is not set
by a scheduled bottom half. In that case we use strerror(-job->ret) as it
was before.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20210225103633.76746-1-sgarzare@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockjob.c

@@ -318,8 +318,12 @@ BlockJobInfo *block_job_query(BlockJob *job, Error **errp)
     info->status    = job->job.status;
     info->auto_finalize = job->job.auto_finalize;
     info->auto_dismiss  = job->job.auto_dismiss;
-    info->has_error = job->job.ret != 0;
-    info->error     = job->job.ret ? g_strdup(strerror(-job->job.ret)) : NULL;
+    if (job->job.ret) {
+        info->has_error = true;
+        info->error = job->job.err ?
+                        g_strdup(error_get_pretty(job->job.err)) :
+                        g_strdup(strerror(-job->job.ret));
+    }
     return info;
 }
 
@@ -356,7 +360,7 @@ static void block_job_event_completed(Notifier *n, void *opaque)
     }
 
     if (job->job.ret < 0) {
-        msg = strerror(-job->job.ret);
+        msg = error_get_pretty(job->job.err);
     }
 
     qapi_event_send_block_job_completed(job_type(&job->job),