get_export_generator() should check argument consistency

[xaav]

when calling bzrlib.export.get_export_generator, an exception should be thrown if one or more of (dest, root) is not set.

This is because the code in bzrlib defaults to dest to set root.

[Martin Pool (mbp)]

I'd rather see the arguments cleaned up a bit; perhaps there should be
multiple factory-type function that take different sets of arguments,
rather than one big function with mutually conflicting arguments.