Apply retries as global, config-level option `downloadRetries` so that
it can be applied to any aptly command which downloads objects.
Unwrap `errors.Wrap` which is used in downloader.
Unwrap `*url.Error` which should be the actual error returned from the
HTTP client, catch more cases, be more specific around failures.
With previous version, `go install` automatically picks up
package `man` and installs `gen.go` as `main` to the $GOPATH/bin which
is not what is expected. Move man page generator to separate
private folder.
