gapis/api: Improve AddGroup error messages.
gapis/api: Improve AddGroup error messages.

pmuetschard/gapid,dsrbecky/gapid,google/gapid,Qining/gapid,google/agi,Qining/gapid,pmuetschard/gapid,baldwinn860/gapid,baldwinn860/gapid,dsrbecky/gapid,Qining/gapid,google/agi,google/agi,Qining/gapid,dsrbecky/gapid,google/gapid,pmuetschard/gapid,google/gapid,baldwinn860/gapid,Qining/gapid,dsrbecky/gapid,google/agi,google/agi,google/gapid,google/gapid,google/agi,Qining/gapid,baldwinn860/gapid,pmuetschard/gapid,dsrbecky/gapid,google/gapid,baldwinn860/gapid,dsrbecky/gapid,baldwinn860/gapid,pmuetschard/gapid,dsrbecky/gapid,google/agi,google/gapid,pmuetschard/gapid,google/gapid,pmuetschard/gapid,pmuetschard/gapid,google/agi,Qining/gapid,Qining/gapid,baldwinn860/gapid,dsrbecky/gapid,baldwinn860/gapid