Investigate possible Rate limiting while running gvfs upgrade

Customer log:
[11/30/2018 1:15:27 PM] TryCheckUpgradeInstallable (Start)
[11/30/2018 1:15:27 PM] TryRunPreUpgradeChecks (Start)
[11/30/2018 1:15:32 PM] Information {"Message":"Successfully finished pre upgrade checks. Okay to run gvfs upgrade."}
[11/30/2018 1:15:32 PM] TryRunPreUpgradeChecks (Stop) {"DurationMs":4630}
[11/30/2018 1:15:32 PM] Information {"Message":"Upgrade is installable."}
[11/30/2018 1:15:32 PM] TryCheckUpgradeInstallable (Stop) {"DurationMs":4649}
[11/30/2018 1:15:32 PM] TryCheckUpgradeAvailable (Start)
[11/30/2018 1:15:33 PM] Error {"Method":"TryFetchReleases","Exception":"System.Net.Http.HttpRequestException: Response status code does not indicate success: 403 (Forbidden).\r\n at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode()\r\n at System.Net.Http.HttpClient.d__33.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at GVFS.Common.ProductUpgrader.TryFetchReleases(List`1& releases, String& errorMessage)","ErrorMessage":"Error fetching release info."}
[11/30/2018 1:15:33 PM] TryCheckUpgradeAvailable (Stop) {"DurationMs":806}
[11/30/2018 1:15:33 PM] Error {"ErrorMessage":"TryRunProductUpgrade: Upgrade checks failed. Network error: could not connect to GitHub(https://api.github.com/repos/microsoft/vfsforgit/releases). Response status code does not indicate success: 403 (Forbidden)."}

The 403 can be possible rate limiting from GitHub. Add logging of rate-limiting info available in Github API response.

More info: https://developer.github.com/v3/#rate-limiting

Start at GVFS.Common.ProductUpgrader.TryFetchReleases, where the GitHub releases request and 403 error are reported. Inspect the GitHub API response handling and add logging for the rate-limit information it provides. Done means a rate-limited response exposes that information while preserving the existing upgrade error.