More accurate parse error messages

Because of the backtracking, we lose the previous point of failure.
The solution is to store the furthest point the parser has parsed to,
when backtracking, and using it when showing parse errors.

lib/less/parser.js

@@ -47,6 +47,7 @@ less.Parser = function Parser(env) {
     var input,       // LeSS input string
         i,           // current index in `input`
         j,           // current chunk
+        furthest,    // furthest index the parser has gone to
         chunks,      // chunkified input
         current,     // index of current chunk, in `input`
         inputLength,
@@ -180,7 +181,7 @@ less.Parser = function Parser(env) {
         parse: function (str, callback) {
             var root, start, end, zone, line, buff = [], c, error = null;
 
-            i = j = current = 0;
+            i = j = current = furthest = 0;
             chunks = [];
             input = str.replace(/\r\n/g, '\n');
             inputLength = input.length;
@@ -225,6 +226,7 @@ less.Parser = function Parser(env) {
             // We split it up into two parts (the part which parsed,
             // and the part which didn't), so we can color them differently.
             if (i < input.length - 1) {
+                i = furthest;
                 start = (function () {
                     for (var n = i; n > 0; n--) {
                         if (input[n] === '\n') { break }
@@ -694,6 +696,7 @@ less.Parser = function Parser(env) {
                     return new(tree.Ruleset)(selectors, rules);
                 } else {
                     // Backtrack
+                    furthest = i;
                     i = memo;
                 }
             },
@@ -714,6 +717,7 @@ less.Parser = function Parser(env) {
                     if ($(this.end)) {
                         return new(tree.Rule)(name, value);
                     } else {
+                        furthest = i;
                         i = memo;
                     }
                 }