Werk #11491: Nagios: Fix broken config reason displaying

This issue is only affecting Checkmk installations that use the Nagios core. In case a configuration error occurs, the reason for this issue should be displayed in the console or the GUI like this: "When Validating Nagios configuration...ERROR: REASON".

However, since release v1.6.0p4 the output of the config validation was not shown to the user. Instead this text was shown instead: "Validating Nagios configuration...ERROR: Configuration Error: local variable 'cmk' referenced before assignment"

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