Error message if multiple exits are performed at the same time

[andreipope]

If multiple exits are performed at the same time, plasma-cli spits the following error message:

Error: replacement fee too low (version=4.0.12)

It should wait for each to get submitted or we could make a command exitCoins <1> <2> <3>
that would send the exit requests under the hood sequentially.

Functionality is not affected.

[gakonst]

This is a similar issue to #19,

Agree with an API for multiple exits.