Example:

err := fmt.Errorf("Cannot read the file!")
log.Printf("Reading %s: %v", file, err)

According to Error Strings at github.com/golang, error strings should not be capitalized or end with punctuation because they might appear among other context.

To fix the format, you can apply the Fix error string format quick-fix. After the quick-fix is applied, the error string will look like this:

err := fmt.Errorf("cannot read the file")
log.Printf("Reading %s: %v", file, err)