Structuring code that shells out to an external script
Overall looks like a good start, some thoughts I had:


If I were implementing this class I probably wouldn't just store the output as a string, I'd parse it in a way that callers would be able to use it effectively.  For example, does it output total time, data transferred, files copied, etc.?  If you're going to try to use this data, I think this class (or a class used by this class) should have the responsibility to process that data, not the caller.
Some prefer the system() method over in-line backticks, for readability, but that's preference.
Never raise Exception, either raise RuntimeError by omitting the first parameter to raise or fail, or create a new exception class (with RuntimeError as a parent) for your purpose.  Similarly you should (in almost all cases) never rescue Exception.
In one of the codebases I work on were have an entire class dedicated to executing commands locally.  One reason we do this is that you can't isolate stdout from stderror using backticks or system(), so we use the popen4 library and generate a Result class that has the exit code, stdout and stderr.  The stderr may be useful if you're trying to relay to the caller what failed exactly.  The caller may want to retry a timeout, but not an authentication issue, for example.


I don't know how widely used or important this class is for your company, so feel free to file any of these under "over-engineering" as appropriate :-).