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

[YuLad]

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?

[ilya-g]

Are you sure NLogConfigurationException should be suppressed when throwExceptions is false? It goes against NLog Exception Handling Rules introduced in v2.0.
If you are looking the way to fix issue #403 (application crashes on configuration reload if it encounters config error), the better approach would be to handle NLogConfigurationError only when reloading configuration, not on initial load.

[YuLad]

@ilya-g I didn't encounter the exception handling rules while going through the wiki, thanks for the pointers. I think the latest commit should fix this issue, but is there anything else I might have missed?