Better global exception handling

Save error now shows a Save as button
Recent list now not changed at every access (only on actual file open)
Some code refactoring

ModernKeePass/Exceptions/SaveException.cs

@@ -0,0 +1,14 @@
+using System;
+
+namespace ModernKeePass.Exceptions
+{
+    public class SaveException : Exception
+    {
+        public new Exception InnerException { get; }
+
+        public SaveException(Exception exception)
+        {
+            InnerException = exception;
+        }
+    }
+}