Allow callback to be optional with pluginify

This fixes the shim to make the callback argument to be optional. For
example, this is valid steal:

```js
steal('can', 'app');
```

Fixes #31
Merge pull request #31 from matthewp/no-callback

Pluginify shim should allow callback to be optional
Prevent swallowing of errors

Previously we were swallowing errors that occurred during the
multi-build process. This adds a test of what happens when we try to
load a module that doesn't exist. The result should be that the promise
is rejected, bubbling all the way up to the user. This fixes it. Fixes
Merge pull request #64 from matthewp/errors

Prevent swallowing of errors