Set the "found" property of syntax errors produced by "error" to null

The property doesn't correspond to anything in the error's message.

lib/compiler/passes/generate-js.js

@@ -1079,12 +1079,7 @@ function generateJS(ast, options) {
       '  function error(message, location) {',
       '    location = location !== void 0 ? location : peg$computeLocation(peg$savedPos, peg$currPos)',
       '',
-      '    throw peg$buildException(',
-      '      message,',
-      '      null,',
-      '      input.substring(peg$savedPos, peg$currPos),',
-      '      location',
-      '    );',
+      '    throw peg$buildException(message, null, null, location);',
       '  }',
       '',
       '  function peg$literalExpectation(text, ignoreCase) {',

lib/parser.js

@@ -534,12 +534,7 @@ function peg$parse(input, options) {
   function error(message, location) {
     location = location !== void 0 ? location : peg$computeLocation(peg$savedPos, peg$currPos)
 
-    throw peg$buildException(
-      message,
-      null,
-      input.substring(peg$savedPos, peg$currPos),
-      location
-    );
+    throw peg$buildException(message, null, null, location);
   }
 
   function peg$literalExpectation(text, ignoreCase) {

spec/behavior/generated-parser-behavior.spec.js

@@ -1218,7 +1218,7 @@ describe("generated parser behavior", function() {
 
               expect(parser).toFailToParse("a", {
                 message:  "a",
-                found:    "a",
+                found:    null,
                 expected: null,
                 location: {
                   start: { offset: 0, line: 1, column: 1 },
@@ -1240,7 +1240,7 @@ describe("generated parser behavior", function() {
               expect(parser).toFailToParse("a", {
                 message:  "a",
                 expected: null,
-                found:    "a",
+                found:    null,
                 location: {
                   start: { offset: 1, line: 1, column: 2 },
                   end:   { offset: 2, line: 1, column: 3 }