misc/cgo/testshared: add format specifier in Errorf calls

Found by cmd/vet

Change-Id: I29dd207ecd40fe703054e8ad4e81b3267ca89da2
Reviewed-on: https://go-review.googlesource.com/17160Reviewed-by: David Crawshaw <crawshaw@golang.org>

misc/cgo/testshared/shared_test.go

@@ -285,23 +285,23 @@ func readNotes(f *elf.File) ([]*note, error) {
 				if err == io.EOF {
 					break
 				}
-				return nil, fmt.Errorf("read namesize failed:", err)
+				return nil, fmt.Errorf("read namesize failed: %v", err)
 			}
 			err = binary.Read(r, f.ByteOrder, &descsize)
 			if err != nil {
-				return nil, fmt.Errorf("read descsize failed:", err)
+				return nil, fmt.Errorf("read descsize failed: %v", err)
 			}
 			err = binary.Read(r, f.ByteOrder, &tag)
 			if err != nil {
-				return nil, fmt.Errorf("read type failed:", err)
+				return nil, fmt.Errorf("read type failed: %v", err)
 			}
 			name, err := readwithpad(r, namesize)
 			if err != nil {
-				return nil, fmt.Errorf("read name failed:", err)
+				return nil, fmt.Errorf("read name failed: %v", err)
 			}
 			desc, err := readwithpad(r, descsize)
 			if err != nil {
-				return nil, fmt.Errorf("read desc failed:", err)
+				return nil, fmt.Errorf("read desc failed: %v", err)
 			}
 			notes = append(notes, &note{name: string(name), tag: tag, desc: string(desc), section: sect})
 		}