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.

system/t02_config/ConfigShowTest_gold

@@ -2,6 +2,7 @@
     "rootDir": "${HOME}/.aptly",
     "downloadConcurrency": 4,
     "downloadSpeedLimit": 0,
+    "downloadRetries": 5,
     "architectures": [],
     "dependencyFollowSuggests": false,
     "dependencyFollowRecommends": false,