Werk #12283: Service discovery page: Do not fail in case of unknown manual check rules

Component	Setup
Title	Service discovery page: Do not fail in case of unknown manual check rules
Date	Mar 4, 2021
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.1.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 2.0.0 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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