lang: Make scope error messages be more consistent

lang/interpret_test/TestAstFunc1/graph8.txtar

@@ -6,4 +6,4 @@ if true {
 	$b = true
 }
 -- OUTPUT --
-# err: errSetScope: variable b not in scope
+# err: errSetScope: var `$b` does not exist in this scope

lang/interpret_test/TestAstFunc2/class-include-as-class0.txtar

@@ -16,7 +16,8 @@ panic($i0.x != "goodbye")
 panic($i1.y != "hello")
 
 # the really tricky case
-# XXX: works atm, but not supported for now: could not set scope: variable i0.y not in scope
+# XXX: works atm, but not supported for now, error is:
+# could not set scope: var `$i0.y` does not exist in this scope
 # We currently re-export anything in the parent scope as available from our
 # current child scope, which makes this variable visible. Unfortunately, it does
 # not have the correct dependency (edge) present in the Ordering system, so it

lang/interpret_test/TestAstFunc2/clear-env-on-var.txtar

@@ -7,4 +7,4 @@ $f = func($x) {
 $name = $f("foo")
 test "${name}" {}
 -- OUTPUT --
-# err: errSetScope: variable x not in scope
+# err: errSetScope: var `$x` does not exist in this scope

lang/interpret_test/TestAstFunc2/import-scope-classes4.txtar

@@ -14,7 +14,8 @@ test "${f1.z}" {}
 test "${f1.x1}" {}
 
 # the really tricky case
-# XXX: works atm, but not supported for now: could not set scope: variable f1.x2 not in scope
+# XXX: works atm, but not supported for now, error is:
+# could not set scope: var `$f1.x2` does not exist in this scope
 # We currently re-export anything in the parent scope as available from our
 # current child scope, which makes this variable visible. Unfortunately, it does
 # not have the correct dependency (edge) present in the Ordering system, so it

lang/interpret_test/TestAstFunc2/import-scope-vars2-fail.txtar

@@ -11,4 +11,4 @@ $x = "this is x.mcl"
 import "x.mcl" as f
 $y = $f.x + " and this is y.mcl"
 -- OUTPUT --
-# err: errSetScope: variable g.x not in scope
+# err: errSetScope: var `$g.x` does not exist in this scope