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/lib.py

@@ -98,6 +98,7 @@ class BaseTest(object):
         "rootDir": "%s/.aptly" % os.environ["HOME"],
         "downloadConcurrency": 4,
         "downloadSpeedLimit": 0,
+        "downloadRetries": 5,
         "architectures": [],
         "dependencyFollowSuggests": False,
         "dependencyFollowRecommends": False,