v0.16.0 snapshot problem

[wjdfx]

Describe the bug
When run command: iris snapshot --home /mnt/data/irisnet/ show error:

panic: load db failed

After testing found the /mnt/data/irisnet/ folder just has 644 permission, but for run snapshot operation.
I think read permission is enough.

Even if permission is required, the error should be a permission problem, not a panic problem.

[dreamer-zq]

@wjdfx Read and write permissions are also required, but this error prompt is indeed not elegant