Error messages about undefined identifiers -> Use variables instead

[schanzer]

Teachers and students have no idea what this error means:

In our Pyret meeting 11/2/23, we agreed to change "identifier" to the less-correct but more-practical "variable"

[blerner]

I'm really not a fan of misusing variable in this context. After talking to @schanzer , we are trying out "name" instead, in contexts that rephrase the sentences slightly to avoid awkward duplication of the word "name"/"named".