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.go

@@ -11,6 +11,7 @@ type ConfigStructure struct { // nolint: maligned
 	RootDir                string                           `json:"rootDir"`
 	DownloadConcurrency    int                              `json:"downloadConcurrency"`
 	DownloadLimit          int64                            `json:"downloadSpeedLimit"`
+	DownloadRetries        int                              `json:"downloadRetries"`
 	Architectures          []string                         `json:"architectures"`
 	DepFollowSuggests      bool                             `json:"dependencyFollowSuggests"`
 	DepFollowRecommends    bool                             `json:"dependencyFollowRecommends"`

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"+