Readd 'Could not create admin.' to the beginning of the error message

was removed in b63ff1b50b7bde0c8f1f95988d076dda63f41fed

Had a good idea how to solve it, but a fix the Erlang Code wouldn't be the best fit (https://github.com/apache/couchdb/blob/master/src/couchdb/couch_httpd.erl#L621) 

See also the discussion in https://github.com/apache/couchdb/pull/167