Fixed problem when original exception message was lost.

[wesolowskim]

*Issue #351 * When the exception was thrown with operation name the original errorMessage was lost. For example

 throw new CfnGeneralServiceException("Operation", new RuntimeException("something happend");

results in "Error occurred during operation 'Operation'." message. If operation parameter would not be passed then the message would contain "something happend".

Description of changes: I changed how Exceptions are constructed so behavior is consistent. If "operation" is passed in the original message is preserved.

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