"assigning to immutable upvar" error is jargony

[catamorphism]

A borrowck error says "assigning to immutable upvar". Not everyone knows what an "upvar" is. It would be better if the error message was less terse: something like "Assigning to immutable variable declared in an enclosing scope". If any other error messages mention "upvar", those should be improved too.

[pcwalton]

Agreed, pushed a fix.

[msullivan]

So this is fixed, right? I'm closing it.