Werk #4560: Fix implausible error msg from windows agent updater

Component	Agent bakery
Title	Fix implausible error msg from windows agent updater
Date	Jun 7, 2017
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.5.0i1 Checkmk Pro, Checkmk Ultimate MT

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