Loader edge case: possible to load a file multiple times

[krzywon]

There is a potential issue with extracting data from a failed loader. IIRC there is a loop over possible formats somewhere above which terminates if a loader runs to completion, but continues to the next loader if an exception is raised. Depending on the available loaders, this may result in the same data being loaded twice.

Within a GUI this should be okay since the user should see the error in the log and be able to see that the data is not what they expect, but the results could be strange if running in a batch script.

Originally posted by @pkienzle in #37 (comment)