toolstash: errors output should match errors guarded

The errors guarded against in the `if` conditions are not the errors
printed with`log.Fatalf()` — instead, they are the same errors copied
from the immediately-preceding error checks.
Maybe some sort of copy-paste mistake.

Reasoning:
- The error printed is always `nil` at the changed lines (`err1`).
- The error printed should be a non-`nil` error (`err2`).

Change-Id: I5e4c1e6a460fb37d9d8652b727e641db2245e3c5
GitHub-Last-Rev: 77a4e86342
GitHub-Pull-Request: golang/tools#489
Reviewed-on: https://go-review.googlesource.com/c/tools/+/580235
Reviewed-by: Joedian Reid <joedian@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tim King <taking@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>

cmd/toolstash/main.go

@@ -423,7 +423,7 @@ func sameObject(file1, file2 string) bool {
 			log.Fatalf("reading %s: %v", file1, err1)
 		}
 		if err2 != nil {
-			log.Fatalf("reading %s: %v", file2, err1)
+			log.Fatalf("reading %s: %v", file2, err2)
 		}
 		if c1 != c2 {
 			return false
@@ -450,7 +450,7 @@ func skipVersion(b1, b2 *bufio.Reader, file1, file2 string) bool {
 			log.Fatalf("reading %s: %v", file1, err1)
 		}
 		if err2 != nil {
-			log.Fatalf("reading %s: %v", file2, err1)
+			log.Fatalf("reading %s: %v", file2, err2)
 		}
 		if c1 != c2 {
 			return false
@@ -473,7 +473,7 @@ func skipVersion(b1, b2 *bufio.Reader, file1, file2 string) bool {
 			log.Fatalf("reading %s: %v", file1, err1)
 		}
 		if err2 != nil {
-			log.Fatalf("reading %s: %v", file2, err1)
+			log.Fatalf("reading %s: %v", file2, err2)
 		}
 		if c1 != c2 {
 			return false