Improve error messages when ClientId or ClientSecret are missing
Improve error messages when ClientId or ClientSecret are missing

Previously the error message would say "Could not find method buildConfigField() for arguments [String, CLIENT_ID, ...". This change makes it clearer what the error is by displaying "Could not get unknown property 'ClientId'" as error message.

Closes #659

edyesed/gh4a,edyesed/gh4a,slapperwan/gh4a,slapperwan/gh4a