Clearer error message when a class extends an interface

https://closure-compiler-debugger.appspot.com/#input0%3D%252F**%2520%2540interface%2520*%252F%250Aclass%2520MyInterface%2520%257B%257D%250A%250Aclass%2520MyClass%2520extends%2520MyInterface%2520%257B%257D%26input1%26conformanceConfig%26externs%26refasterjs-template%26includeDefaultExterns%3D1%26CHECK_SYMBOLS%3D1%26CHECK_TYPES%3D1%26LANG_IN_IS_ES6%3D1%26MISSING_PROPERTIES%3D1%26PRESERVE_TYPE_ANNOTATIONS%3D1%26PRETTY_PRINT%3D1%26TRANSPILE%3D1

Currently this gives two different errors, and one of them mentions `$jscomp.inherits` which is totally unhelpful and meaningless to the user. We should:
1. Only report one error
2. Make sure it's clear
3. (bonus) Suggest that the user probably meant to include an `@implements` annotation in their JSDoc, rather than writing `extends`