fix: formatter validation message

[alexandear]

Before:

$ go run ./cmd/golangci-lint fmt -E formatter
Error: can't load config: formatter is a formatter
Failed executing command with error: can't load config: formatter is a formatter
exit status 3

After:

$ go run ./cmd/golangci-lint fmt -E formatter
Error: can't load config: formatter is not a formatter
Failed executing command with error: can't load config: formatter is not a formatter
exit status 3

[ldez]

LGTM