[cef] Append pipeline errors to error.message

[andrewkroh]

What does this PR do?

If the decode_cef Beat processor has errors then those errors are
lost if there is a pipeline error.

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.

Related

• Relates: [cef] Fleet CEF Integration not handling CEF version 2 #2676

[elasticmachine]

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

[elasticmachine]

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Build stats

• Start Time: 2022-03-08T17:41:09.356+0000

• Duration: 14 min 20 sec

Test stats 🧪

Test	Results
Failed	0
Passed	84
Skipped	0
Total	84

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