Mediation fee safety mechanism blocks tiny payments

[karlb]

We disallow payments where the fees exceed MAX_MEDIATION_FEE_PERC (currently 0.2%). When users try to send tiny amounts, this will fail due to the mediator's flat fees. There are two problems with this:

• The error message is uninformative (there is no route). Solved by Add link to docs for fee limit error [skip tests] #6199?
• There is no way to force the client to do the payment, anyway

See also #6188.