wasm::ReportTier2ResultsOffThread can get called with `success == false` in the case where a compilation has been cancelled. It then constructs an error string claiming OOM, which is confusing (and just plain wrong). This patch just passes a `cancelled` flag to ReportTier2ResultsOffThread, with which it can construct the correct error string. Differential Revision: https://phabricator.services.mozilla.com/D250190