refactor(apiv2): rename APIErrors to Errors, move IsUserError to helper method

PluralKit.API/Startup.cs

@@ -140,10 +140,7 @@ namespace PluralKit.API
                 var exc = ctx.Features.Get<IExceptionHandlerPathFeature>();
 
                 // handle common ISEs that are generated by invalid user input
-                if (
-                       (exc.Error is InvalidCastException && exc.Error.Message.Contains("Newtonsoft.Json"))
-                    || (exc.Error is FormatException && exc.Error.Message.Contains("was not recognized as a valid DateTime"))
-                )
+                if (exc.Error.IsUserError())
                 {
                     ctx.Response.StatusCode = 400;
                     await ctx.Response.WriteAsync("{\"message\":\"400: Bad Request\",\"code\":0}");