Reduce Unauthenticated Errors in Sentry (#238)

* Updated README to be explicit that kavita.db needs to be writable.

* Implemented a new Exception type that is for throwing a message to UI without logging in Sentry.

API/Extensions/ClaimsPrincipalExtensions.cs

@@ -1,4 +1,5 @@
 using System.Security.Claims;
+using Kavita.Common;
 
 namespace API.Extensions
 {
@@ -6,7 +7,9 @@ namespace API.Extensions
     {
         public static string GetUsername(this ClaimsPrincipal user)
         {
-            return user.FindFirst(ClaimTypes.NameIdentifier)?.Value;
+            var userClaim = user.FindFirst(ClaimTypes.NameIdentifier);
+            if (userClaim == null) throw new KavitaException("User is not authenticated");
+            return userClaim.Value;
         }
     }
 }