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 %")

It check for engine folder for matching version, and if none is found, then 
it download full list from website. The list is cached for 10 minute in case user were trying to re-check it again since re-retrieving the list have network delay.