Add helpful error message if dist is not a string in gf_dist()

[rpruim]

I'm thinking something like this:

gf_dist()
## Error in gf_dist() : You must specify a distribution.
gf_dist(dist = dbinom)
## Error in gf_dist(dist = dbinom) : 
##   `dist' must be a string naming a distribution; don't forget the quotes.