"error.cert.not.found" and "error.explicit-sign-no-cert" errors duplicate each other. They are emitted one after another from the same function [1].

We don't need two duplicate error messages about the missing certificate.

This is a legacy to keep ErrorTest pass. JDK-8355328 added test cases for these errors to the ErrorTest. Back then, it made some sense, as these errors were emitted by separate functions (though it still doesn't make sense to report the same error twice). Now, after the refactoring, when both errors come from the same function, it doesn't anymore.

[1] https://github.com/openjdk/jdk/blob/3f07710270dbe7268f21828dff20e2eb810b1e70/src/jdk.jpackage/macosx/classes/jdk/jpackage/internal/SigningIdentityBuilder.java#L149