The error message tells you pretty explicitly
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what the error is
C:\Users\dev\AppData\Local\Xamarin\Android.Support.v4\21.0.3\android_m2repository_r10.zip is not a valid zip file
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How to fix it
Please download https://dl-ssl.google.com/android/repository/android_m2repository_r10.zip and extract it to the C:\Users\dev\AppData\Local\Xamarin\Android.Support.v4\21.0.3\content directory.