Bug 924858 - Part 1: Add |append| mode flag to OS.File.open on Unix. r=yoric

To match previous behavior, this mode flag will default to |true|.
Bug 924858 - Part 2: Implement |append| mode for OS.File.open on Windows. r=yoric
Bug 926635 - Improve layout of uncaught Promise.jsm error messages. r=Yoric