[System] Fix missing field mappings

[legoguy1000]

What does this PR do?

Fix missing mappings for error.code and error.message.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes Possibly System Integration Mapping Issue #4071

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• Start Time: 2022-08-28T12:06:06.674+0000

• Duration: 15 min 47 sec

Test stats 🧪

Test	Results
Failed	0
Passed	273
Skipped	0
Total	273

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (3/3)	💚
Files	100.0% (4/4)	💚 2.896
Classes	100.0% (4/4)	💚 2.896
Methods	60.759% (48/79)	👎 -28.926
Lines	98.793% (2702/2735)	👍 7.042
Conditionals	100.0% (0/0)	💚

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Pinging @elastic/security-external-integrations (Team:Security-External Integrations)