requests for cores which had known startup init failures should result in 500 not 404

SOLR-3591 added support for tracking if a core failed to init properly, and reporting this data back in STATUS request to the CoreAdminHandler so they could be displayed in the UI.

Attempts to use those cores anyway (for queries or updates, etc...) by users/clients that may not realize the core failed to init results in 404 errors because the core doesn't exist, however it should be fairly straight forward to intead return a 500 error wrapping the cause of the init failure.