[Bugfix:RainbowGrades] ensure directory is writable (#4184)
[Bugfix:RainbowGrades] ensure directory is writable (#4184)


RCOS-Grading-Server/HWserver,RCOS-Grading-Server/HWserver,Submitty/Submitty,Submitty/Submitty,Submitty/Submitty,RCOS-Grading-Server/HWserver,RCOS-Grading-Server/HWserver,Submitty/Submitty,RCOS-Grading-Server/HWserver,RCOS-Grading-Server/HWserver,Submitty/Submitty,Submitty/Submitty,Submitty/Submitty,RCOS-Grading-Server/HWserver,RCOS-Grading-Server/HWserver,Submitty/Submitty,Submitty/Submitty,RCOS-Grading-Server/HWserver