Werk #3150: fixed helpers failing to start and high cpu load when an unparsable local check is installed

When python fails to parse a check file (local or shipped), the check_mk helpers would produce an erro message and exit. check_mk would then restart the helper, repeating the process infinitely which left check_mk in a non-working state, causing high cpu load and quickly growing the cmc.log.

This only occurs when installing syntactically broken checks so most users shouldn't be affected.

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