[TT-9231] Fix go plugin error responses

[PatrickTaibel]

Description

Don't send an error message if the go plugin itself did that already.

Related Issue

#5141

Motivation and Context

The change of #4922 causes Tyk to always send an error message to the client when an error within a go plugin occurs. In case the error is caused by a sent status code >= 400 by the plugin itself this is not desiderable as it will append the Tyk error message to an already existing response from the plugin.

How This Has Been Tested

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring or add test (improvements in base code or adds test coverage to functionality)

Checklist

• I ensured that the documentation is up to date
• I explained why this PR updates go.mod in detail with reasoning why it's required
• I would like a code coverage CI quality gate exception and have explained why

[lghiur]

@PatrickTaibel thanks for the PR. We've realised internally that this was caused by a recent change we've done, so we handled the fix in a different PR #5562.

Closing this one for now, and thanks for the contribution.