Fix #283: throwExceptions ="false" but Is still an error

Altered conditions in the MustBeRethrown extension method.
Fixed relevant tests and added a new one.

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I'm unsure of what to do with the `ConditionParseException`:

 - should it be thrown even if `throwException="false"`?
 - should it be sub-classed from `NLogConfigurationException` and swallowed as well?