Error strings follow a set of guidelines to ensure uniformity and good composability.

Quoting Go Code Review Comments:

    Error strings should not be capitalized (unless beginning with
    proper nouns or acronyms) or end with punctuation, since they are
    usually printed following other context. That is, use
    fmt.Errorf("something bad") not fmt.Errorf("Something bad"), so
    that log.Printf("Reading %s: %v", filename, err) formats without a
    spurious capital letter mid-message.