Better error status for the engine non availability

[vletoux]

On the engine page (/engines/list), there is a status (which can be refreshed and the engine queried)

In the case below, there is only "Error" written.
You don't know if the engine is down, if there is a problem with the output of the engine, a proxy error, an authentication error, ...

I suggest to create the following status:

• if you received a proxy error, return Proxy Denied (such as connection terminated)
• if you receive a network error (such as port close), return Connection Denied
• if you have a time out, return "TimeOut" or "Unavailable"
• if there is an authentication error with the plugin (401 or 403 error), return "Not authenticated"
• if you receive a data, but which cannot be interpreted, return "Data error"