add error case for dist operations

fermi.go

@@ -171,6 +171,9 @@ func operateDistsAsSamples(dist1 Dist, dist2 Dist, op string) (Dist, error) {
 			zs[i] = xs[i] + ys[i]
 		case "-":
 			zs[i] = xs[i] - ys[i]
+		default:
+			fmt.Println("Error: Operation not recognized")
+			return nil, errors.New("Operation not recognized")
 		}
 	}