GuestLog - User message improvements

- Added UserMessageService to have common methods for showing messages and errors to the user.
- Added Result record so services can return status on operation error (so it can be shown in the UI messages).
- Updated services to use the new Result record.
- Updated pages to use the new IUserMessageService.

src/Somnia.GuestLog/Models/ResultCode.cs

@@ -0,0 +1,11 @@
+namespace Somnia.GuestLog.Models;
+
+public static class ResultCode
+{
+    public const string Ok = "Ok";
+    public const string UnknownError = "UnknownError";
+    public const string FailedValidation = "FailedValidation";
+    public const string NotFound = "NotFound";
+
+    public const string GuestCurrentlyInStay = "GuestCurrentlyInStay";
+}