girder_client: Improve local_folder validation for upload command
girder_client: Improve local_folder validation for upload command

Upload requires the file to exist and be readable

girder/girder,RafaelPalomar/girder,sutartmelson/girder,manthey/girder,girder/girder,jbeezley/girder,sutartmelson/girder,manthey/girder,data-exp-lab/girder,jbeezley/girder,Kitware/girder,girder/girder,kotfic/girder,manthey/girder,RafaelPalomar/girder,adsorensen/girder,girder/girder,adsorensen/girder,data-exp-lab/girder,RafaelPalomar/girder,kotfic/girder,data-exp-lab/girder,Xarthisius/girder,Xarthisius/girder,kotfic/girder,Xarthisius/girder,data-exp-lab/girder,manthey/girder,RafaelPalomar/girder,data-exp-lab/girder,sutartmelson/girder,Kitware/girder,adsorensen/girder,Kitware/girder,jbeezley/girder,jbeezley/girder,kotfic/girder,RafaelPalomar/girder,sutartmelson/girder,Xarthisius/girder,adsorensen/girder,kotfic/girder,Kitware/girder,sutartmelson/girder,Xarthisius/girder,adsorensen/girder