Support friends endpoint

@leahsteinberg 
This error should be really more descriptive. It looks like we're doing the same thing in `fetchFriendWithExternalId`, so we should fix both.

In `VENCore.h`, I'd add a new error code:
```
NS_ENUM(NSInteger, VENCoreErrorCode) {
    VENCoreErrorCodeNoDefaultCore,
    VENCoreErrorCodeNoAccessToken,
    VENCoreErrorCodeMissingParameter
};
```

Then, in `fetchFriendsWithExternalId` and `fetchFriendWithExternalId`, we can return 

```
[NSError errorWithDomain:VENErrorDomainCore code:VENCoreErrorCodeMissingParameter description:@"Missing parameter externalId" recoverySuggestion:@"Please pass a valid externalId"];
```