improve error message when group column is missing

[alexg9010]

Right now we fail like this:

KeyError in line 193 of /gnu/store/0fjyszb91qqki545krh0j3244932c1c7-pigx-chipseq-0.0.51/libexec/pigx_chipseq/scripts/Check_Config.py:
'Group'
  File "/gnu/store/0fjyszb91qqki545krh0j3244932c1c7-pigx-chipseq-0.0.51/libexec/pigx_chipseq/Snake_ChIPseq.py", line 47, in <module>
  File "/gnu/store/0fjyszb91qqki545krh0j3244932c1c7-pigx-chipseq-0.0.51/libexec/pigx_chipseq/scripts/Check_Config.py", line 37, in validate_config
  File "/gnu/store/0fjyszb91qqki545krh0j3244932c1c7-pigx-chipseq-0.0.51/libexec/pigx_chipseq/scripts/Check_Config.py", line 193, in check_settings
  File "/gnu/store/0fjyszb91qqki545krh0j3244932c1c7-pigx-chipseq-0.0.51/libexec/pigx_chipseq/scripts/Check_Config.py", line 193, in <listcomp>

This should become a more descriptive error.