Invalid `GIDConfiguration` object reports unsupported URL error, rather than something more descriptive

## Background

I'm loading the `clientID` key from an `.xcconfig` file into an `AuthManager`, which is responsible for creating the `GIDConfiguration` object.

```swift
// AuthManager.swift
var config = GIDConfiguration(clientID: "asdf")

init() {
// Fetches the API token from an .xcconfig thru the Info.plist
if let clientID = Bundle.main.object(forInfoDictionaryKey: "GID_CLIENT_ID") as? String {
config = GIDConfiguration(clientID: clientID)
}
}
```
However, if no code is provided, I receive a runtime crash with the following error:
```
Your app is missing support for the following URL schemes: asdf'
terminating with uncaught exception of type NSException
```

## Suggestion

I'd hope that for new users this would have a more detailed explanation, such as "Invalid GIDconfiguration object; key was invalid". "Missing URL scheme support' to me would imply an issue with the `AppDelegate` rather than a config object.

I recognize this is totally minor (especially given a lot of other API design issues I've experienced in v6 with SwiftUI) but I figured I might as well report it :)