Add command help for /save, /delete, /saveroom and /loadroom

Add 2nd optional parameter to HedgewarsScriptLoad. If false, its OK if file does not exist. Also, return true/false on success

Backed out changeset a62408ee8281. Allowing HedgewarsScriptLoad to not stop if script is missing is dumb

Allow script to be missing when calling HedgewarsScriptLoad, return true/false based on success

Don't load locale Lua script if LOCALE == "en"

Add description of hwengine command-line arguments in --help

Critical error if loading a script has failed