Fix some format string errors. (#188)

Author: dlorenc

differs/differs.go

@@ -66,7 +66,7 @@ func (req DiffRequest) GetDiff() (map[string]util.Result, error) {
 
 	var err error
 	if len(results) == 0 {
-		err = fmt.Errorf("Could not perform diff on %s and %s", img1, img2)
+		err = fmt.Errorf("Could not perform diff on %v and %v", img1, img2)
 	} else {
 		err = nil
 	}
@@ -90,7 +90,7 @@ func (req SingleRequest) GetAnalysis() (map[string]util.Result, error) {
 
 	var err error
 	if len(results) == 0 {
-		err = fmt.Errorf("Could not perform analysis on %s", img)
+		err = fmt.Errorf("Could not perform analysis on %v", img)
 	} else {
 		err = nil
 	}

tests/integration_test.go

@@ -209,7 +209,7 @@ func TestDiffAndAnalysis(t *testing.T) {
 			args = append(args, "-j")
 			actual, stderr, err := runner.Run(args...)
 			if err != nil {
-				t.Fatalf("Error running command: %s. Stderr: ", err, stderr)
+				t.Fatalf("Error running command: %s. Stderr: %s", err, stderr)
 			}
 			e, err := ioutil.ReadFile(test.expectedFile)
 			if err != nil {