Improve validation of group size and threshold input parameters

[nkuba]

When requested keep with group size 1 it is accepted by the factory contract but the client fails in a loop with:

2020-04-13 08:01:04.270632 ERROR keep-ecdsa node.go:174: failed to generate threshold signer: [group should have at least 2 members but got: [1]]

To do:

• update BondedECDSAKeepFactory to require group size > 0 and threshold > 0 - Requiring group size and threshold to be > 0 #410
• the client should check requested groupSize before starting the retry loop, log an error if group size < 2 (we will implement it proper way later in Implement single signer support #408) and return - Catch unsupported keep size and threshold in client #415