fix: `tns platform add ios` should not be executed on non-macOS
fix: `tns platform add ios` should not be executed on non-macOS

`tns platform add ios` should not be executed on non-macOS system. Currently in the master branch it exits silently, add correct error, so the user will know that it is not okay.

NathanaelA/nativescript-cli,NathanaelA/nativescript-cli,NathanaelA/nativescript-cli,NativeScript/nativescript-cli,NativeScript/nativescript-cli,NativeScript/nativescript-cli,NativeScript/nativescript-cli,NathanaelA/nativescript-cli