Make Server connection status more useful

Problem/Motivation

When you go to /admin/config/search/search-api/server/your_server_machine_name, the Search API Server Summary table has a Connection row which displays:

• "The ElasticSearch cluster was reached successfully" if the server is available and can be pinged, xor,
• "The ElasticSearch cluster could not be reached. Further data is therefore unavailable." if the server is available, but cannot be pinged

This is okay, but it would be more useful if:

1. If the server is available and can be pinged, it would be good to know...
   1. What the cluster thinks that it is named
   2. What versions that the Elasticsearch Nodes in the cluster are is running
   3. What the cluster thinks that its overall status is

   ...as this information would help a site administrator know if they connected to the correct cluster.

2. If the server is available but cannot be pinged, it would be good to know...
   1. If there was an error establishing the connection
   2. What HTTP status code the ping responded with

   ...as this information would help a site administrator know if they made a mistake in the URL (i.e.: connection error), with their credentials (i.e.: 400 error), or if the server is having some sort of issue (i.e.: 500 error).

Proposed resolution

Update the displayed messages. Update internal APIs to make it easier to get the information we need.

Remaining tasks

1. Write a patch
2. Review and feedback
3. RTBC and feedback
4. Commit
5. Release

User interface changes

Likely only changes to the text in the UI. This will affect translations.

API changes

To be determined: likely only API additions.

Data model changes

To be determined: likely none.