Provide a better error when proxies.json is invalid

#### Investigative information

I have a function that I am trying to use to mock some API.

I kept getting this user facing error:

`Function host is not running.`

Then I decided to create an actual function which would not start which then said that I could not start it.

```
The function runtime is unable to start. Microsoft.Azure.AppService.Proxy.Common: Expression 2561097253 error: body is empty.
Session Id: 9d4e2602fbfa4e34a6bed8794a1c958c

Timestamp: 2018-07-24T23:16:39.491Z
```

#### Repro steps

Create a function. Add a proxy config which is malformed. User facing error should say `Proxy configuration invalid.`

#### Expected behavior
I would expect that the online proxy editor would not allow me to get myself into a bad state where the file is not good. As soon as I make I change, I would expect the editor to tell me, no, this change will not fly.

#### Actual behavior
Cryptic error. `Function host is not running.` ,
`body is empty`
None of these mention that they are failing because the json is malformed or invalid.

#### Related information
https://github.com/Azure/azure-functions-host/issues/2680

No source file or test is named in the issue. Start by reproducing the failure with a malformed proxies.json and trace the proxy configuration handling; done means the user-facing error clearly identifies the configuration as invalid instead of reporting only that the function host is not running.

Written by the indexing model from the issue text.