`rake elasticsearch:import:model` prints errors

Prior to this pull-request, these errors would be silently ignored. This caused me some confusion, since when I saw `[IMPORT] Done` I assumed that the task had completed successfully. Now the number of errors is printed:

```
$ bundle exec rake environment elasticsearch:import:model CLASS=User
[IMPORT] 4 errors occurred
[IMPORT] Done
```

It would be nice to have more details about the errors (I can submit another PR later). But this is an improvement over failing silently.