OpenDataset: add prettyish labels

Added a nicer title to the OpenDataset command, along with several of
the options. Also includes a header message with the current dataset.
Fix OpenDataset behavior

There are two changes in this patch:
* removed superfluous @Parameter labels. Parameters are called by name.
* Propagated exceptions when relevant.

The DatasetIOPlugin should now behave more reasonably, returning null if
there was cancelation of the OpenDataset command, and chaining the
IOException if one was thrown.