Better Handling for offers which have no direct channel

[fredo]

Problem

Every offer which gets broadcasted and matches is currently taken by the taker. There is no testing if it would be even possible to exchange via the current raiden network. (i.e. no path to target)

Furthermore there is no handling of what happens if payments cannot be send. This needs to be taken into account.

Possible Solutions

• Filter offers by direct connections (short term solution)
• make it possible to open a channel to a particular maker
• Error handling if there is no direkt path / recognition of addresses who failed to send payments