Fix #419, improve Cannot index message (sortof)

jv_get() doesn't know if it's being called in the context of a jq
program or not, so it can't produce a very useful error message when the
types of the to-be-indexed value and the key don't agree.  For now,
including the key (when it is a short string) in the error message is as
significant an improvement as is easy to make.