Error handling for "Could not find file 'C:\home\site\wwwroot'" on host startup

For host startup failures like "System.IO.FileNotFoundException : Could not find file 'C:\home\site\wwwroot'." it may be better to immediately exit the process instead of continuously retrying as invocations are sometimes routed to the role instance causing further issues.

Stack trace:

```
System.IO.FileNotFoundException : Could not find file 'C:\home\site\wwwroot'.
at System.IO.__Error.WinIOError(Int32 errorCode,String maybeFullPath)
at System.IO.Directory.InternalCreateDirectory(String fullPath,String path,Object dirSecurityObj,Boolean checkHost)
at System.IO.Directory.InternalCreateDirectoryHelper(String path,Boolean checkHost)
at System.IO.Directory.CreateDirectory(String path)
at System.IO.Abstractions.DirectoryWrapper.CreateDirectory(String path)
at Microsoft.Azure.WebJobs.Script.FileUtility.EnsureDirectoryExists(String path) at C:\projects\azure-webjobs-sdk-script\src\WebJobs.Script\Extensions\FileUtility.cs : 97
at Microsoft.Azure.WebJobs.Script.ScriptHost.InitializeFileSystem() at C:\projects\azure-webjobs-sdk-script\src\WebJobs.Script\Host\ScriptHost.cs : 571
at Microsoft.Azure.WebJobs.Script.ScriptHost.PreInitialize() at C:\projects\azure-webjobs-sdk-script\src\WebJobs.Script\Host\ScriptHost.cs : 551
at Microsoft.Azure.WebJobs.Script.ScriptHost.Initialize() at C:\projects\azure-webjobs-sdk-script\src\WebJobs.Script\Host\ScriptHost.cs : 292
at Microsoft.Azure.WebJobs.Script.ScriptHostManager.RunAndBlock(CancellationToken cancellationToken) at C:\projects\azure-webjobs-sdk-script\src\WebJobs.Script\Host\ScriptHostManager.cs : 178
```

Example IcM: 397469698

Start with ScriptHost.InitializeFileSystem in src/WebJobs.Script/Host/ScriptHost.cs and the EnsureDirectoryExists call in FileUtility.cs, then trace how ScriptHostManager.RunAndBlock handles startup failures. Confirm the retry behavior for the missing wwwroot case and define completion as exiting rather than repeatedly retrying; no test file is named in the issue.

Written by the indexing model from the issue text.