better error message when there is no parameter after 'show'

src/main.zig

@@ -19,7 +19,9 @@ pub fn main() !void {
             Base32Error.InvalidCharacter, Base32Error.InvalidPadding => {
                 try std.io.getStdErr().writer().print("The secret is invalid.\n", .{});
             },
-            ArgumentError.UnknownParameter => {
+            ArgumentError.UnknownParameter, //
+            ArgumentError.MissingAuthenticatorParam,
+            => {
                 // do nothing, error message is already written (because the message contains the name of the unknown parameter)
             },
             ArgumentError.InvalidOtpAuthUrl => {},
@@ -164,7 +166,7 @@ fn zeroPad(allocator: Allocator, digits: u4, x: anytype) ![]u8 {
     return result;
 }
 
-const ArgumentError = error{ InvalidOtpAuthUrl, UnknownParameter, MissingConfigLocation, AuthenticatorNotFound };
+const ArgumentError = error{ InvalidOtpAuthUrl, UnknownParameter, MissingConfigLocation, AuthenticatorNotFound, MissingAuthenticatorParam };
 
 fn parseArgs(allocator: Allocator, args: []const []const u8) !Args {
     var result = Args{
@@ -187,7 +189,8 @@ fn parseArgs(allocator: Allocator, args: []const []const u8) !Args {
         } else if (eql(u8, "show", arg)) {
             i += 1;
             if (i >= args.len) {
-                return error.MissingConfigLocation;
+                std.debug.print("expected authenticator name after command 'show'\n  get a list of available authenticators with 'zig-totp list'\n", .{});
+                return error.MissingAuthenticatorParam;
             }
             result.show = args[i];
         } else {