Better error-messages for trades

Improved error messages for trades.  Also added an experimental fix for https://github.com/Jessecar96/SteamBot/issues/598
Sorry, force of habit.  Though to be fair, there is no "current convention" in the SteamBot codebase - it's a wild amalgam of every coding style imaginable.

But you're right, we should try to keep it consistent on a per-file basis, at least.