split `file` param validation error into two: folder error and filename error

[phuongquan]

Easy to miss that the problem is that the folder doesn't exist rather than that the filename is invalid

[phuongquan]

Bit tricky as current param validation mechanism (validate_param_byname()/validate_param()) only accommodates a single validation function and single static error message. Would probably either want to call validate_param() twice or allow a list of validation/error messages.