chore: update to use DigitalData.Core.Exceptions instead of project exceptions classes

EnvelopeGenerator.GeneratorAPI/Middleware/ExceptionHandlingMiddleware.cs

@@ -1,6 +1,6 @@
 namespace EnvelopeGenerator.GeneratorAPI.Middleware;
 
-using EnvelopeGenerator.Application.Exceptions;
+using DigitalData.Core.Exceptions;
 using Microsoft.AspNetCore.Http;
 using Microsoft.Extensions.Logging;
 using System.Net;