Add verification errors to output of VerifyTimestampAuthority

[codysoyland]

Summary

This is an alternative to #325 which modifies the return value of VerifyTimestampAuthority to include verification errors separately from non-verification errors.

Release Note

Documentation

[steiza]

I like this! It'll definitely help us surface non-critical failures to users so they can do debugging or better understand what is going on, like for #48.

[haydentherapper]

Thanks for working on two approaches. I like this one more, since it fits well into how we are currently verifying bundles. We can also update Rekor to do the same.