XL: Refactor usage of reduceErrs and consistent behavior. (#2240)

This refactor is also needed in lieu of our quorum
requirement change for the newly understood logic behind
klauspost/reedsolom implementation.

xl-v1_test.go

@@ -73,7 +73,7 @@ func TestCheckSufficientDisks(t *testing.T) {
 		},
 		// Lesser than minimum number of disks < 6.
 		{
-			disks[0:5],
+			disks[0:3],
 			errXLMinDisks,
 		},
 		// Odd number of disks, not divisible by '2'.