polkit: reword 'Interactive authentication required' error

I guess the current wording of "Interactive authentication required." is
hard to grok for many users. Let's try to reword this, and say
explicitly:

1. That this is a form of "access denied" error
2. That interactive auth could remedy this
3. But that the client disabled interactive auth

I think these are the three primary elements the error msg needs to
convey. I tried to distill this in a short error string with this.

Fixes: #2081

src/shared/bus-polkit.c

@@ -323,7 +323,8 @@ static int async_polkit_read_reply(sd_bus_message *reply, AsyncPolkitQuery *q) {
         } else if (challenge) {
                 log_debug("Polkit authorization for action '%s' requires interactive authentication, which we didn't allow.", a->action);
                 q->error_action = TAKE_PTR(a);
-                sd_bus_error_set_const(&q->error, SD_BUS_ERROR_INTERACTIVE_AUTHORIZATION_REQUIRED, "Interactive authentication required.");
+                sd_bus_error_set_const(&q->error, SD_BUS_ERROR_INTERACTIVE_AUTHORIZATION_REQUIRED,
+                                       "Access denied as the requested operation requires interactive authentication. However, interactive authentication has not been enabled by the calling program.");
         } else {
                 log_debug("Polkit authorization for action '%s' denied.", a->action);
                 q->denied_action = TAKE_PTR(a);