Crash when an release asset doesn't exist

Currently, the script crashes whenever a release asset is unable to download (for example a 404 response). This change instead logs the failure and allows the script to continue. No retry logic is enabled, but at least it prevents the crash and allows the backup to complete. Retry logic can be implemented later if wanted.

closes https://github.com/josegonzalez/python-github-backup/issues/129

bin/github-backup

@@ -569,15 +569,27 @@ def download_file(url, path, auth):
     request.add_header('Accept', 'application/octet-stream')
     request.add_header('Authorization', 'Basic '.encode('ascii') + auth)
     opener = build_opener(S3HTTPRedirectHandler)
-    response = opener.open(request)
 
-    chunk_size = 16 * 1024
-    with open(path, 'wb') as f:
-        while True:
-            chunk = response.read(chunk_size)
-            if not chunk:
-                break
-            f.write(chunk)
+    try:
+        response = opener.open(request)
+
+        chunk_size = 16 * 1024
+        with open(path, 'wb') as f:
+            while True:
+                chunk = response.read(chunk_size)
+                if not chunk:
+                    break
+                f.write(chunk)
+    except HTTPError as exc:
+        # Gracefully handle 404 responses (and others) when downloading from S3
+        log_info('Skipping download of asset {0} due to HTTPError: {1}'.format(url, exc.reason))
+    except URLError as e:
+        # Gracefully hadnle other URL errors
+        log_info('Skipping download of asset {0} due to URLError: {1}'.format(url, e.reason))
+    except socket.error as e:
+        # Gracefully handle socket errors
+        # TODO: Implement retry logic
+        log_info('Skipping download of asset {0} due to socker error: {1}'.format(url, e.strerror))
 
 
 def get_authenticated_user(args):