Issue with PXD000002

PXD000002 is available in [ProteomeCentral](http://proteomecentral.proteomexchange.org/cgi/GetDataset?ID=PXD000002) but not found in PRIDE, even though the hosting repository is PRIDE. 

Trying to access it with `rpx`:
```r
> PXDataset("PXD000002")
Querying ProteomeXchange for PXD000002.
Error in open.connection(con, "rb") : HTTP error 400.
```