AutoHost_commands.cs: improve !setengine command

Added following feature:
1) guess full version name (ie: !setengine 97.0.1-255 --into--> 97.0.1-255-githash)
2) add 2 additional error message (ie: "No such engine version" and "Engine downloading. 0 %")

If engine version do not match anything in engine folder, it download full list from website. The list is cached for 10 minute in case user were trying to re-check random version or re-check download progress of requested version since re-retrieving the list have unnecessary delay.