test: do not send header before processing data

[kruskall]

Do not write status header before processing data or it could lead to
flaky tests due to a race when reading apmServerInternals.Data.
Add proper logging and return early if an error was returned.

[ghost]

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• Start Time: 2022-08-31T15:47:03.954+0000

• Duration: 8 min 33 sec

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Test	Results
Failed	0
Passed	160
Skipped	2
Total	162

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