Mount paths should be required when starting Robustar with robustar.sh script.

[PaulCCCCCCH]

Thanks @zehongl4 for pointing out this issue.

Users can either start Robustar with the launcher or the scripts. If users want to start with a script, they can follow the templates provided in scripts/example.sh, which calls scripts/robustar.sh with a bunch of parameters. However, they sometimes miss some of the parameters (e.g., they specified all the paths except the training path -t, which is not unlikely because there are so many paths to mount).

The problem is, in robustar.sh, we define default values for many parameters see here. When some input parameters are missing, these default values will cause errors that look like cannot bind with relative path, which makes users confused.

A solution is to remove the default values and make these parameters required. If any of them are missing, a prompt will be displayed.