Failed bribe should cost resources

[elexis]

With the bribe button we get information when we successfully bribe a trader of a player, but we also get information when the bribing fails (the fact that the player doesn't have any trade).

For example in a diplomacy game we might have many enemies and trade might be very important. With this button we can test for free whether our enemies still don't have trade yet.

So I propose to make a non-sucessful bribe cost something like 1/4th of the price.

[elexis]

In r19967:

Add a cost for unsuccessful bribe attempts.

Patch By: mmoanis

Reviewed By: mimo

Trac Tickets: #4623

Differential Revision: ​​https://code.wildfiregames.com/D624

Thanks!