Simplify generated error unmarshalling code in json protocol based clients

[apache-hb]

Motivation and Context

This reduces the size of the generated client jars by about ~1%, as an example the acm client jar is 0.8% smaller but this scales with the number of operations a service has.

elliothb@ELLIOT-SERVER:~/github/aws-sdk-java-v2-elliothb$ ls -lh acm-before.jar acm-after.jar 
-rw-r--r-- 1 elliothb elliothb 844K Apr  9 12:22 acm-after.jar
-rw-r--r-- 1 elliothb elliothb 850K Apr  9 12:21 acm-before.jar

Modifications

This reduces the amount of code generated for error unmarhsalling in json protocol clients by moving the error response handlers body into a helper method instead of generating a copy in each operation handler.

Testing

• I did a full rebuild and ran unit tests.

Checklist

• 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
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.
• 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