Werk #17070: Agent controller on Linux: More informative error message in case of file reading errors in import mode

On Linux systems, the agent controller always runs as the cmk-agent user, independently of the user starting the controller. Hence, in import mode, the input file must readable by the cmk-agent user. Otherwise, the import will fail. As of this werk, the controller displays a more informative error message in this case, hinting at the point that the input file must be readable by cmk-agent.

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