If downloading the extension bundle takes too long, the runtime doesn't start properly

If you're on a slow Internet connection such that the extension bundle can't download within about 2 minutes, then the runtime fails.

Manually downloading the extension bundle (nearly 5 minutes on my connection) and expanding it where the ExtensionBundleManager looks for it works around the issue.

Console logs:
```
[hostname]:[project] [username]$ func start

%%%%%%
%%%%%%
@ %%%%%% @
@@ %%%%%% @@
@@@ %%%%%%%%%%% @@@
@@ %%%%%%%%%% @@
@@ %%%% @@
@@ %%% @@
@@ %% @@
%%
%

Azure Functions Core Tools (2.7.1585 Commit hash: 9a2b7239b937148e3b76e6f7999798629a82134a)
Function Runtime Version: 2.0.12641.0
[9/19/19 8:14:17 PM] Building host: startup suppressed:False, configuration suppressed: False
[9/19/19 8:14:17 PM] Looking for extension bundle Microsoft.Azure.Functions.ExtensionBundle at /var/folders/pl/qgvhtxwx7153kf2c8y9bc8kw0000gn/T/Functions/ExtensionBundles/Microsoft.Azure.Functions.ExtensionBundle
[9/19/19 8:14:17 PM] Fetching information on versions of extension bundle Microsoft.Azure.Functions.ExtensionBundle available on https://functionscdn.azureedge.net/public/ExtensionBundles/Microsoft.Azure.Functions.ExtensionBundle/index.json
[9/19/19 8:14:18 PM] Downloading extension bundle from https://functionscdn.azureedge.net/public/ExtensionBundles/Microsoft.Azure.Functions.ExtensionBundle/1.0.0/Microsoft.Azure.Functions.ExtensionBundle.1.0.0.zip to /var/folders/pl/qgvhtxwx7153kf2c8y9bc8kw0000gn/T/5d2fd1c0-d8f7-4ced-82c9-93cf0de7f0bc/Microsoft.Azure.Functions.ExtensionBundle.1.0.0.zip
Hosting environment: Production
Content root path: /Users/[username]/[project]
Now listening on: http://0.0.0.0:7071
Application started. Press Ctrl+C to shut down.
Value cannot be null.
Parameter name: provider
Application is shutting down...
[9/19/19 8:16:31 PM] Stopping host...
[9/19/19 8:16:31 PM] Host shutdown completed.
```

Start with the ExtensionBundleManager referenced in the report and trace the extension-bundle download shown in the console logs. Reproduce with a connection that takes longer than two minutes, then verify that the runtime starts successfully after the bundle download completes.

Written by the indexing model from the issue text.