Bug 949308 - Add the magic cookie for signaling a failure, r=philor

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extra : rebase_source : 08490e9bb29a44afe61418c94d3da79da33f4b0d
bug 945313: Check for content length when downloading files and retry if not the same. r=aki
No bug - Update env var for mocha reporter. r=aki
Bug 949740 - Dump emulator log if unable to telnet; r=armenzg