Model errors when using relations with the MongoDB connector cause crashes

Nobody has claimed this yet.

I have relations setup with a model which includes a property similar to this:

```
versions: [
{
id: String
}
]
```

If the `versions` property is actually an object, rather than an array (due to an error in saving), loopback crashes when it attempts to resolve relations involving this model.

This is the error I get:

```
/Users/zackbloom/s/API/node_modules/mongodb/lib/mongodb/connection/base.js:245
throw message;
^
Error: Items must be an array: [object Object]
at new List (/Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/list.js:25:11)
at /Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/model.js:178:39
at Array.forEach (native)
at Function.ModelClass.forEachProperty (/Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/model-builder.js:271:51)
at ModelConstructor.ModelBaseClass._initProperties (/Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/model.js:164:8)
at /Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/dao.js:557:13
at Array.forEach (native)
at /Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/dao.js:554:12
at /Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/include.js:59:11
at /Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/node_modules/async/lib/async.js:113:21
at /Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/node_modules/async/lib/async.js:24:16
at /Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/include.js:124:9
at /Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/node_modules/async/lib/async.js:111:13
at Array.forEach (native)
at _each (/Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/node_modules/async/lib/async.js:32:24)
at Object.async.each (/Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/node_modules/async/lib/async.js:110:9)
at processIncludeItem (/Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/include.js:106:11)
at /Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/include.js:57:5
at /Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/node_modules/async/lib/async.js:111:13
at Array.forEach (native)
at _each (/Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/node_modules/async/lib/async.js:32:24)
at Object.async.each (/Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/node_modules/async/lib/async.js:110:9)
at Function.Inclusion.include (/Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/include.js:56:9)
at Function.f (/Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/jutil.js:100:15)
at Function.f [as include] (/Users/zackbloom/s/API/node_modules/loopback-datasource-juggler/lib/jutil.js:100:15)
at /Users/zackbloom/s/API/node_modules/loopback-connector-mongodb/lib/mongodb.js:519:33
at /Users/zackbloom/s/API/node_modules/mongodb/lib/mongodb/cursor.js:163:16
at commandHandler (/Users/zackbloom/s/API/node_modules/mongodb/lib/mongodb/cursor.js:709:16)
at /Users/zackbloom/s/API/node_modules/mongodb/lib/mongodb/db.js:1846:9
at Server.Base._callHandler (/Users/zackbloom/s/API/node_modules/mongodb/lib/mongodb/connection/base.js:445:41)
at /Users/zackbloom/s/API/node_modules/mongodb/lib/mongodb/connection/server.js:468:18
at [object Object].MongoReply.parseBody (/Users/zackbloom/s/API/node_modules/mongodb/lib/mongodb/responses/mongo_reply.js:68:5)
at [object Object]. (/Users/zackbloom/s/API/node_modules/mongodb/lib/mongodb/connection/server.js:426:20)
at [object Object].EventEmitter.emit (events.js:95:17)
at [object Object]. (/Users/zackbloom/s/API/node_modules/mongodb/lib/mongodb/connection/connection_pool.js:201:13)
at [object Object].EventEmitter.emit (events.js:98:17)
at Socket. (/Users/zackbloom/s/API/node_modules/mongodb/lib/mongodb/connection/connection.js:339:18)
at Socket.EventEmitter.emit (events.js:95:17)
at Socket. (_stream_readable.js:746:14)
at Socket.EventEmitter.emit (events.js:92:17)
at emitReadable_ (_stream_readable.js:408:10)
at emitReadable (_stream_readable.js:404:5)
at readableAddChunk (_stream_readable.js:165:9)
at Socket.Readable.push (_stream_readable.js:127:10)
at TCP.onread (net.js:528:21)
```

The error itself isn't terrible, but I'd certainly rather it be returned through the HTTP request, rather than crashing the entire app.

I've tried wrapping parts of that stack in a `try` block, but it looks like the mongodb connector does some fancy magic with `nextTick` to bypass the actual stack.

Start with the relation-inclusion path in lib/include.js and the property initialization in lib/model.js, then inspect the connector callback in lib/mongodb.js. Reproduce a relation lookup where versions is an object instead of an array; done means the failure is returned through the HTTP request rather than crashing the application.

Written by the indexing model from the issue text.