feat(apollo): map unprocessable entity code to bad user input

[kishieel]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

The code in error extensions is INTERNAL_SERVER_ERROR if we set HttpStatus.UNPROCESSABLE_ENTITY for ValidationPipe.

{
  "errors": [
    {
      "extensions": {
        "code": "INTERNAL_SERVER_ERROR",
      }
    }
  ]
}

What is the new behavior?

The code in error extensions is BAD_USER_INPUT.

{
  "errors": [
    {
      "extensions": {
        "code": "BAD_USER_INPUT",
      }
    }
  ]
}

Does this PR introduce a breaking change?

• Yes
• No

Other information

Can we have it merged please or there is some reason why this mapping was not added before?

[kostnerek]

Nice PR, thought about it somę time ago, just didnt have time to make it! @kamilmysliwiec look and it

[kamilmysliwiec]

lgtm