refine "no servers hosting shard" wording

[cpoerschke]

"no servers hosting shard" can be inaccurate and misleading/confusing e.g. there could well be active and healthy servers hosting the shard but the request parameters filter to not match them.

"no eligible servers hosting shard" could be alternative wording for the "filters don't match" case and the remaining cases could be "no active servers hosting shard" wording.

TODO: create https://issues.apache.org/jira/browse/SOLR-XXXXX