Issue error if no char exists or if target char doesn't exist

[arthur-shaw]

This PR closes #28 by:

• Checking if ANY char exists
• Checking if the target char exists

[arthur-shaw]

@kbjarkefur , two things:

1. Does this modest addition look right? For example, would you style the error message differently? SMCL newb here.
2. Do you see a better way to check whether ANY char exists? Currently, the code uses ds, has(char) to look for any variable with a char. Since char list does not have a return value I can inspect, the ds solution is the only one I see, even though I have some reservations about its performance for dsets with lots of columns.