Nil error can show missing value #41
Description
A NoMethodError can show the variable that is nil by post-parsing the filter/eval string and testing the values. That would speed up eval debugging for users.
r.tumor.dp>=20 and r.normal.dp>=20 and r.info.fb_ssc>=20 and qual>=10
(eval):4:in call_cached_gfilter': undefined method>=' for nil:NilClass (NoMethodError)