improve error message on service length

[rkarthick]

Currently if len(service name) > 63, the API incorrectly reports the issue to be "not a valid RFC 1123" misleading the developer. Changed the wording to ensure it reflects the correct logic

[hashicorp-cla]

All committers have signed the CLA.

[lgfa29]

Thanks for the fix @rkarthick 🙂

While reviewing the PR I noticed that the same error message was being returned by ValidateName, but I think that we were not using it because when it's called from the structs package it's passed a version of the service name with some place holders for environment variables.

So rather than copying over the message, I changed the method to not add the service name and leave that up to the caller.

[github-actions]

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