[Stack Monitoring] add error.message mapping

[klacabane]

Summary

Add mapping for the error.message field.
It will allow SM to filter erroneous documents for elastic/kibana#140102 and #4011

[elasticmachine]

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• Start Time: 2022-09-06T23:38:39.074+0000

• Duration: 14 min 9 sec

Test stats 🧪

Test	Results
Failed	0
Passed	62
Skipped	0
Total	62

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[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (0/0)	💚
Files	100.0% (0/0)	💚 2.809
Classes	100.0% (0/0)	💚 2.809
Methods	49.462% (46/93)	👎 -39.933
Lines	100.0% (0/0)	💚 9.059
Conditionals	100.0% (0/0)	💚

[matschaffer]

Ahhh. Nice find.