Improve error messages for invalid bridge/bond configuration

[jnohlgard]

Pull Request

What? (description)

Improve the details in the error message shown when the machine config contains an interface that has more than one 'master' link.

Why? (reasoning)

I was experimenting with some changes in the config code and I saw these messages because of mistakes in my code, but the messages contained too little detail to allow me to pinpoint what to fix without using the debugger to look at the variables.

Acceptance

Please use the following checklist:

• you linked an issue (if applicable)
• you included tests (if applicable)
• you ran conformance (make conformance)
• you formatted your code (make fmt)
• you linted your code (make lint)
• you generated documentation (make docs)
• you ran unit-tests (make unit-tests)

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[smira]

thank you!