Buck should print a warning message for non-exist flavors

Created by: v-jizhang

A repro can be found in #2399 (closed). A wrong build command buck build :Test#dwarf-with-dsym,iphoneos-arm64 (which should be buck build :Test#dwarf-and-dsym,iphoneos-arm64) was entered but buck does not complain about that. Make buck display a warning.