Fixed line/column numbers in math expr/ops error messages. Updated tests.

lib/less/tree/rule.js

@@ -42,6 +42,12 @@ tree.Rule.prototype = {
                               this.merge,
                               this.index, this.currentFileInfo, this.inline);
         }
+        catch(e) {
+            if (e.index === undefined) {
+                e.index = this.index;
+            }
+            throw e;
+        }
         finally {
             if (strictMathBypass) {
                 env.strictMath = false;