app_voicemail: Emit warning if asking for nonexistent mailbox.

Currently, if VoiceMailMain is called with a mailbox, if that
mailbox doesn't exist, then the application silently falls back
to prompting the user for the mailbox, as if no arguments were
provided.

However, if a specific mailbox is requested and it doesn't exist,
then no warning at all is emitted.

This fixes this behavior to now warn if a specifically
requested mailbox could not be accessed, before falling back to
prompting the user for the correct mailbox.

ASTERISK-29920 #close

Change-Id: Ib4093b88cd661a2cabc5d685777d4e2f0ebd20a4

apps/app_voicemail.c

@@ -11512,10 +11512,16 @@ static int vm_execmain(struct ast_channel *chan, const char *data)
 		else
 			ast_copy_string(vms.username, args.argv0, sizeof(vms.username));
-		if (!ast_strlen_zero(vms.username) && (vmu = find_user(&vmus, context ,vms.username)))
-			skipuser++;
-		else
+		if (!ast_strlen_zero(vms.username)) {
+			if ((vmu = find_user(&vmus, context ,vms.username))) {
+				skipuser++;
+			} else {
+				ast_log(LOG_WARNING, "Mailbox '%s%s%s' doesn't exist\n", vms.username, context ? "@": "", context ? context : "");
+				valid = 0;
+			}
+		} else {
 			valid = 0;
+		}
 	}
 	if (!valid)