Update section about exceptions

[melnikovi]

Purpose of this pull request

Update section about exceptions in technical guidelines.

Affected DevDocs pages

• https://devdocs.magento.com/guides/v2.3/coding-standards/technical-guidelines.html

Links to Magento source code

• ...

whatsnew
Added a technical guideline (clause 5.20) about how to handle exceptions in observers and plugins.

[magento-cicd2]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
2 out of 3 committers have signed the CLA.

✅ jeff-matthews
✅ dshevtsov
❌ melnikovi

[devops-devdocs]

An admin must run tests on this PR before it can be merged.

[dshevtsov]

Please consider splitting into two statements and avoiding passive voice and nested semantical recursions:

Plugins MUST throw only exceptions declared by the method to which the plugins are added.
Observers MUST throw only exceptions declared by the method that triggers an event.

Discussed with @melnikovi .

[erikmarr]

Looks good! Just a minor word order swap.

[jeff-matthews]

running tests

[dshevtsov]

@jeff-matthews this pull request still requires a review form an architect @maghamed .

[jeff-matthews]

Thanks @dshevtsov. I forgot that all architect reviewers must approve; not just one.

Waiting for your approval @maghamed.

[dshevtsov]

running tests

[dshevtsov]

running tests

[dshevtsov]

running tests

[ghost]

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