Error message when no decoys provided

[percolator]

When provided with target but no decoy PSMs, percolator halts after training with a cryptic error message: "Exception caught: Error : the input data has too good separation between target and decoy PSMs."

WSN pointed out that: "I think you should modify Percolator to terminate with an error message ("No decoys provided") prior to training. That would be a lot more understandable to the end user than telling them you got perfect separation."