FIX Don't mark mocked response as an error

[GuySartorelli]

This response type is only ever returned from the findgridfield action on the GridFieldQueuedExportButton (see GridFieldQueuedExportButton::handleAction()). The only way this action is accessed in normal operation is from the GenerateCSVJob::getGridField() method, which uses a mock request via Director::test() to get the response. More discussion about why this happens is in the parent issue.

In other words, the only way this response class is ever used, it cannot be a 500 error. It's always going to be a success - unless some exception is thrown, in which case this response won't be used anyway.

My best guess as to why this was set to 500 is in case some smart-alek decides to manually access the findgridfield action by typing in the appropriate URL to do so.... which is both a super weird edge case and also not something that explicitly needs a 500 response. A 200 response is, I'd argue, more appropriate even in that scenario, since the action has done what it's meant to do. There was no error.

tl;dr: 500 was probably never correct for this in the first place.

Issue

• Module breaks on Silverstripe 4.12 #76

[emteknetnz]

Tested locally, works good. The 500 in the constructor looks really weird and I cannot think of any reason to keep it, so happy to remove.