API patch improvements

- add PatchObject.CheckIsValid
- use transaction when creating member, as to not create a member if the
patch is invalid
- return edited system in `PATCH /s` endpoint

PluralKit.Core/Models/Patch/PatchObject.cs

@@ -1,7 +1,17 @@
-namespace PluralKit.Core
+using System;
+
+namespace PluralKit.Core
 {
+
+    public class InvalidPatchException : Exception
+    {
+        public InvalidPatchException(string message) : base(message) {}
+    }
+
     public abstract class PatchObject
     {
         public abstract UpdateQueryBuilder Apply(UpdateQueryBuilder b);
+
+        public void CheckIsValid() {}
     }
 }