Description of problem:
Unexpected errors can happen during drift detection while the agent is generating a change set report file. The file on disk could be in an inconsistent state causing errors for future drift detection.

This problem manifests itself by following the steps for bug 736204. 

When the drift detector runs for the configuration having an invalid base directory, an exception is thrown, and an empty change set file is left on the file system. For example, for a resource with id 123 and a drift configuration named, BAD_CONFIG, the file <RHQ_AGENT_HOME>/data/changesets/123/BAD_CONFIG/changeset.txt would be empty. That file should never be empty, even with a valid base directory. The file should always contain headers.

When an unexpected error occurs during drift detection, the changeset.txt file should be reset to its state prior to the drift detection running. This will ensure that changeset.txt remains in a consistent state from one run of the drift detector to the next. 

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