plot set 3, change "varid" to "varnom"
plot set 5, add an error message and fix a typo
Check for the weird case of the computed variable being entirely masked out.
We've seen this, albeit from a bad data file.
In this case, return None instead of the useless variable.
In method compute_plot_var_value, check for an entirely masked-out variable.
This hasn't actually happened however, because it's getting caught in reduced_variable.reduce().