Updated troubleshoot-apiml-error-codes.md

[ArooshLele]

List the file(s) included in this PR:
troubleshoot-apiml-error-codes.md

Describe your pull request here:
The description of an error message (ZWEAG120E) was to be added in the file.
This error message is ZWEAG120E, and it is related to gateway.
The error message is ZWEAG120E is different from the error message, ZWEAS120E.
The description of the error message ZWEAS120E is already written, and it is related to security.

After creating the PR, follow the instructions in the comments.

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[github-actions]

😺 Thank you for creating this PR! To publish your content to Zowe Docs, follow these required steps.

• Add the label review: doc.
• Identify your content topic with a label. (Examples: area: apiml, area: cli, area: install and config, etc.)
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  • If adding content that needs to be removed from V3 documentation, add the V3 N/A tag.
• Select the master branch if your PR updates content that is on the live site. Select docs-staging if your PR updates content for a future release.
• Notify the Doc Squad about this PR. If you don't know whom should review your content, message the #zowe-doc Slack channel. If you know which Doc Squad writer should approve your content, add that person as a reviewer.

Need help? Contact the Doc Squad in the #zowe-doc Slack channel.

[github-actions]

😺 Thank you for creating this PR! To publish your content to Zowe Docs, follow these required steps.

• Add the label review: doc.
• Identify your content topic with a label. (Examples: area: apiml, area: cli, area: install and config, etc.)
• Specify the major Zowe release(s) for your content. (Examples: release: V1, release: V2, release: V3)
  • If adding content that needs to be removed from V3 documentation, add the V3 N/A tag.
• Select the master branch if your PR updates content that is on the live site. Select docs-staging if your PR updates content for a future release.
• Notify the Doc Squad about this PR. If you don't know whom should review your content, message the #zowe-doc Slack channel. If you know which Doc Squad writer should approve your content, add that person as a reviewer.

Need help? Contact the Doc Squad in the #zowe-doc Slack channel.

[ArooshLele]

FYI only: @gauravs-20 @ArooshLele

[github-actions]

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