[RAILS] Added displaying error count in the import Rake task

Prior to this pull-request, these errors would be silently ignored and users might assume 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, but this is an improvement over failing silently.

Closes #48