tests: lightdb: fix error handling and fix copy-paste error in log

[mniestroj]

This test does not expect any counter, so remove this word from log message.

If there was an error during asynchronous (callback-based) request, then such error was not propagated to struct async_data structure, but instead it was still set to 0, just like the API would be successful. Later, test would discover a problem anyway, as received data would not match with expected one. However, it is better to propagate error and not rely on data comparison.

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