When diagnosing why code signing fails, such as:
> error : No valid iOS code signing keys found in keychain. You need to request a codesigning certificate from https://developer.apple.com.
it would be helpful to know why the existing certificates on the machine (if any) aren't applicable.
"The certificate C is not valid because its prefix P does not match any of the app's prefixes P1, P2, ..."
"The certificate C is not valid because it expired on October 21st, 2015"
Partially implemented: https://github.com/xamarin/Xamarin.MacDev/pull/16
Partially fixed in https://github.com/xamarin/xamarin-macios/pull/2865