Utility.FlattenException is ommitting line numbers from the exception printed

Sample code

```cs
public static Task Run(HttpRequestMessage req, TraceWriter log)
{
throw new System.Exception("boom");
}
```

This displays error

```
[Error] Exception while executing function: Functions.HttpTriggerCSharp1. mscorlib: Exception has been thrown by the target of an invocation. f-HttpTriggerCSharp1__981322258: boom.
```

which is missing the line numbers. If I catch and print the actual exception thrown myself, it shows the correct line number