fix: properly handle http.ErrServerClosed

[kruskall]

Logs API server: the server was ignoring ErrServerClosed and logging an error.

APM Data server: the server was using an equality check with the error message but
it was never satisfied because of a typo ('server closed' vs 'Server closed'), leading
to an error message.

Both servers are now handling the error and logging a message that the server stopped
successfully. The code is using go 1.13 errors to avoid comparing strings and for better
compatibility.

[apmmachine]

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• Start Time: 2022-07-18T14:43:54.013+0000

• Duration: 17 min 37 sec

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Test	Results
Failed	0
Passed	220
Skipped	4
Total	224

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[marclop]

Looks good! Just left a couple of suggestions on rewording the errors and using a non-capitalized error to comply with static analysis tools.