Fix status code mapping for RPCv2

[dagnir]

Fix for RPCv2 error unmarshalling to default to 500 HTTP status code for server side faults, and 400 for non-faults when an explicit HTTP status code is not provided in the model. This default mapping only has an effect when the response being unmarshalled does not have the status code set.

Also makes the code more robust so that new protocols raise a codegen error if the code in ProtocolSpec is not updated.

Motivation and Context

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Testing

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Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

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• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
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• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• I confirm that this pull request can be released under the Apache 2 license

[dagnir]

Successful build blocked on #7426.

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