make GitHub errors inherit from `GitHub::Error`
make GitHub errors inherit from `GitHub::Error`

BelieveC/classroom,hackerkid/classroom,mkcode/classroom,ConnorCartwright/classroom,hackerkid/classroom,mkcode/classroom,BelieveC/classroom,hackerkid/classroom,mkcode/classroom,BelieveC/classroom,jasnow/classroom,ConnorCartwright/classroom,BelieveC/classroom,jasnow/classroom,ConnorCartwright/classroom,jasnow/classroom