Fix error handler in simple extension examples

[andreyvelich]

I fixed problem with error handler in simple extension.
We can use write_error instead of write, so status_code and status_message will be populated.
Also, I added few unit tests for simple_ext1.

cc @Zsailer

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[echarles]

LGTM. Failing CI on build (windows-latest, 3.10 is not related to this PR. Thx @andreyvelich

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