We should output to stderr on EXIT_FAILURE (#2558)

Making sure we use stderr to output the help
message when there is an invalid number of
arguments, or when there's any error related
to the arguments.

src/cli/Add.cpp

@@ -83,7 +83,7 @@ int Add::execute(const QStringList& arguments)
 
     const QStringList args = parser.positionalArguments();
     if (args.size() != 2) {
-        outputTextStream << parser.helpText().replace("keepassxc-cli", "keepassxc-cli add");
+        errorTextStream << parser.helpText().replace("keepassxc-cli", "keepassxc-cli add");
         return EXIT_FAILURE;
     }