Improve error message when EventHub connection string is not set

[lindydonna]

Repro steps:

• Create a new C# event hub function via func new
• Add connection string named bar to function.json
• Run host without setting value for connection string bar in appsettings.json

Expected result:

Error: Missing connection string for "bar". Did you forget to run "func azure functionapp fetch-app-settings <functionAppName>"?

Actual result:

Reading host configuration file 'C:\CDriveCode\FuncCliProjects\new-proj\host.json'
Generating 0 job function(s)
Starting Host (HostId=171ab663c24c47c781e2981bbc8ac788, Version=1.0.0.0, ProcessId=14200, Debug=False)
No job functions found. Try making your job classes and methods public. If you're using binding extensions (e.g. Service
Bus, Timers, etc.) make sure you've called the registration method for the extension(s) in your startup code (e.g. confi
g.UseServiceBus(), config.UseTimers(), etc.).
Job host started
The following 1 functions are in error:
EventHubTriggerCSharp: Value cannot be null.
Parameter name: receiverConnectionString.

[lindydonna]

Closing as it is handled by #38