Surfacing of "Function host is not running" underlying error

[gallivantor]

Currently there are some scenarios which lead to the Function App not working with the message "Function host is not running" being returned if you manually navigate to the function app URL.

However, it is very difficult to debug this and find the root cause, as the actual error does not seem to get surfaced anywhere.

The following don't work:

• Opening the "Log streaming" view - shows nothing in either tab
• Application Insights - in some cases there might be an exception here, but in other cases there isn't

There needs to be an easy way to see what the underlying error message is, when the host won't start up.

[ColbyTresness]

Azure/azure-functions-host#4934 would solve this @fabiocav @jeffhollan