De-capitalise all fmt.Errorf messages
Related issue: #397 (closed)
Update calls to fmt.Errorf to conform to the official CodeReviewComments guide, which says:
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")
Source: https://github.com/golang/go/wiki/CodeReviewComments#error-strings
Edited by Tom Quirk