Bug 909147 - Check client.py-args existence, r=jcranmer

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extra : rebase_source : 3e9bc3dad1114aef24b304e514e6141f454906f1
Bug 906831 - "Intermittent OSError: [Errno 2] No such file or directory: '../../../mozilla/dist/xpi-stage/lightning/components/interfaces.manifest.lck' build failure on c-c" r=Standard8, a=me