Throw an exception in bbs.exec_xtrn() when passed no argument

... or when passed an invalid external program code or number, rather just just returning false. The underlying exec_xtrn() C++ function may return false (failure) for other reasons, so let's not obfuscate the other potential failure causes.

src/sbbs3/js_bbs.cpp

@@ -1351,7 +1351,9 @@ js_exec_xtrn(JSContext *cx, uintN argc, jsval *arglist)
 
 	JS_SET_RVAL(cx, arglist, JSVAL_VOID);
 
-	if(argc) {
+	if(!js_argc(cx, argc, 1))
+		return JS_FALSE;
+
 	if(JSVAL_IS_STRING(argv[0])) {
 		JSVALUE_TO_ASTRING(cx,argv[0],code, LEN_CODE+2, NULL);
 		if(code==NULL)
@@ -1364,11 +1366,10 @@ js_exec_xtrn(JSContext *cx, uintN argc, jsval *arglist)
 		if(!JS_ValueToECMAUint32(cx,argv[0],&i))
 			return JS_FALSE;
 	}
-	}
 
 	if(i>=sbbs->cfg.total_xtrns) {
-		JS_SET_RVAL(cx, arglist, JSVAL_FALSE);
-		return(JS_TRUE);
+		JS_ReportError(cx, "Invalid external program specified");
+		return JS_FALSE;
 	}
 
 	rc=JS_SUSPENDREQUEST(cx);