add warnings for invalid el triggered consolidations & withdrawals

[pk910]

adds warnings for probably invalid requests:

there are warnings for:

• consolidation requests to a validator without 0x02 credentials
• consolidation request from a validator that is younger than the shard committee period (not consolidate-able)
• withdrawal request for a validator that is younger than the shard committee period (not withdrawable)

fixes #229, but keeps the possibility to submit invalid requests for testing purposes.
cc @remyroy

[pk910]

#236 should also make the outcome of requests more clear.