Improve syntax help for step arguments

[erikmannerfelt]

Admittedly, I often mix up the placement of commas when writing step arguments:

> rsgpr -f ... --steps "subset(0, 200)"
Unrecognized step: 200)

This is quite uninformative. It would be nicer to hint toward what may be wrong, for example by just informing that a comma is a delimiter for different steps, not for arguments.