More informative error when staking slots are full

[DavidMinarsch]

First, it would help to explain the user when staking slots are full and a stake transaction fails.

Second, the script should always check first whether any slots are full. The check is very simple: 1) get maxNumberServices from https://gnosisscan.io/address/0x5add592ce0a1b5dcecebb5dcac086cd9f9e3ea5c#readContract#F16 and compare against 2) length of getServiceIds from here https://gnosisscan.io/address/0x5add592ce0a1b5dcecebb5dcac086cd9f9e3ea5c#readContract#F10