improve error message if argument for --config is missing

src/main.zig

@@ -20,7 +20,8 @@ pub fn main() !void {
                 try std.io.getStdErr().writer().print("The secret is invalid.\n", .{});
             },
             ArgumentError.UnknownParameter, //
-            ArgumentError.MissingAuthenticatorParam,
+            ArgumentError.MissingAuthenticatorParam, //
+            ArgumentError.MissingConfigLocation,
             => {
                 // do nothing, error message is already written (because the message contains the name of the unknown parameter)
             },
@@ -62,14 +63,14 @@ fn mainInternal() !void {
 
 fn printHelp() void {
     const msg =
-        \\Usage: zig-totp command [options]
+        \\Usage: zig-totp [command] [options]
         \\
         \\Commands:
         \\  list                   List the configured authentiators
         \\  show NAME              Show the code for the authenticator with name NAME
         \\
         \\Options:
-        \\  --config path          The path to a config file (default is $XDG_CONFIG_HOME/zig-totp or $HOME/.zig-totp).
+        \\  --config [path]        The path to a config file (default is $XDG_CONFIG_HOME/zig-totp or $HOME/.zig-totp).
         \\ 
     ;
     std.debug.print("{s}", .{msg});
@@ -181,6 +182,7 @@ fn parseArgs(allocator: Allocator, args: []const []const u8) !Args {
         if (eql(u8, "--config", arg)) {
             i += 1;
             if (i >= args.len) {
+                std.debug.print("expected a path after parameter '--config'\n  see 'zig-totp' for help\n", .{});
                 return error.MissingConfigLocation;
             }
             result.config_location = args[i];