Add custom exception classes and Extensions folder

Introduce four custom exceptions (BurnAnnotationException, CreateReportException, ExportDocumentException, MergeDocumentException) under EnvelopeGenerator.ServiceHost.Exceptions for improved error handling. Update the project file to include the new Extensions folder.

EnvelopeGenerator.ServiceHost/Exceptions/BurnAnnotationException.cs

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+namespace EnvelopeGenerator.ServiceHost.Exceptions;
+
+public class BurnAnnotationException : ApplicationException
+{
+    public BurnAnnotationException(string message) : base(message)
+    {
+    }
+
+    public BurnAnnotationException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}

EnvelopeGenerator.ServiceHost/Exceptions/CreateReportException.cs

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+namespace EnvelopeGenerator.ServiceHost.Exceptions;
+
+public class CreateReportException : ApplicationException
+{
+    public CreateReportException(string message) : base(message)
+    {
+    }
+
+    public CreateReportException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}

EnvelopeGenerator.ServiceHost/Exceptions/ExportDocumentException.cs

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+namespace EnvelopeGenerator.ServiceHost.Exceptions;
+
+public class ExportDocumentException : ApplicationException
+{
+    public ExportDocumentException(string message) : base(message)
+    {
+    }
+
+    public ExportDocumentException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}

EnvelopeGenerator.ServiceHost/Exceptions/MergeDocumentException.cs

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+namespace EnvelopeGenerator.ServiceHost.Exceptions;
+
+public class MergeDocumentException : ApplicationException
+{
+    public MergeDocumentException(string message) : base(message)
+    {
+    }
+
+    public MergeDocumentException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}