Bug 912918 - Part 1: Code style cleanups in app validator. r=paul

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extra : rebase_source : cb4258aabfd1c7b7a9540048e27a7ab752d79d85
Bug 912918 - Part 2: Display warnings and errors if both are present. r=paul

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extra : rebase_source : fe7a8989c9e09b42b31c76c14cbd08034c5b91f3
Bug 912918 - Part 3: Keep validating after errors when possible. r=paul

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extra : rebase_source : c4241212495876f6539a82f9ce8b5df52463f960