Check for ERRORARRAY before checking its length

If the API client is called with a batch of commands, then
obj.ERRORARRAY does not exist since obj is an array.  We could
deal with the whole batching stuff in linode-api, or just let the
caller deal with it.

In either case, we should be prepared for the case where ERRORARRAY
does not actually exist, so checking its length throws an exception.
Return object if obj.DATA not set

This is also related to using the batch API.  If a caller uses the
batch API, then obj.DATA will not be set since obj is an array.  In
this case, we just pass obj back and let the caller do what they like.

There's probably a better way to deal with this, but this is the
simplest.
Merge pull request #4 from bluesmoon/patch-1

Check for ERRORARRAY before checking its length