check global scope for variable before local

src/scope.rs

@@ -56,9 +56,12 @@ impl Scope {
 
     pub fn get_var(&self, v: &str) -> SassResult<Value> {
         let name = &v.replace('_', "-");
-        match self.vars.get(name) {
-            Some(v) => Ok(v.clone()),
-            None => get_global_var(name),
+        match get_global_var(name) {
+            Ok(v) => Ok(v),
+            Err(e) => match self.vars.get(name) {
+                Some(v) => Ok(v.clone()),
+                None => Err(e)
+            }
         }
     }