qom: Improve error messages when property has no getter or setter

When you try to set a property that has no setter, the error message
blames "insufficient permission":

    $ qemu-system-x86_64 -S -display none -nodefaults -monitor stdio
    QEMU 7.1.50 monitor - type 'help' for more information
    (qemu) qom-set /machine type q35
    Error: Insufficient permission to perform this operation

This implies it could work with "sufficient permission".  It can't.
Change the error message to:

    Error: Property 'pc-i440fx-7.2-machine.type' is not writable

Do the same for getting a property that has no getter.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20221012153801.2604340-2-armbru@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>

qom/object.c

@@ -1383,7 +1383,8 @@ bool object_property_get(Object *obj, const char *name, Visitor *v,
     }
 
     if (!prop->get) {
-        error_setg(errp, QERR_PERMISSION_DENIED);
+        error_setg(errp, "Property '%s.%s' is not readable",
+                   object_get_typename(obj), name);
         return false;
     }
     prop->get(obj, v, name, prop->opaque, &err);
@@ -1402,7 +1403,8 @@ bool object_property_set(Object *obj, const char *name, Visitor *v,
     }
 
     if (!prop->set) {
-        error_setg(errp, QERR_PERMISSION_DENIED);
+        error_setg(errp, "Property '%s.%s' is not writable",
+                   object_get_typename(obj), name);
         return false;
     }
     prop->set(obj, v, name, prop->opaque, errp);