Requiring group size and threshold to be > 0

[dimpar]

Refs: #392

It is not possible to create a group with 0 members. Similarly, it is not possible to create a group with 0 honest threshold. Added appropriate validations to openKeep contract functions.

[pdyraga]

@nkuba Is what we have here enough to close #392? Do we need to add a validation to the client to fail on 1-1 case and scenarios not currently supported by tss-lib?

[pdyraga]

Ah, alright. I see the second checkbox in the issue:

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 #408) and return.

Let's address it in a separate PR.