Rework HTTP downloader retry logic

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.

utils/config_test.go

@@ -57,6 +57,7 @@ func (s *ConfigSuite) TestSaveConfig(c *C) {
 		"  \"rootDir\": \"/tmp/aptly\",\n"+
 		"  \"downloadConcurrency\": 5,\n"+
 		"  \"downloadSpeedLimit\": 0,\n"+
+		"  \"downloadRetries\": 0,\n"+
 		"  \"architectures\": null,\n"+
 		"  \"dependencyFollowSuggests\": false,\n"+
 		"  \"dependencyFollowRecommends\": false,\n"+