Error handling: Extract exception building into its own function

The exception-creating code will get somewhat hairy soon, so let's make
sure them mess will be contained.

Implements part of #198.

lib/compiler/passes/generate-javascript.js

@@ -909,7 +909,8 @@ module.exports = function(ast, options) {
     '      peg$maxFailExpected.push(expected);',
     '    }',
     '',
-    '    function peg$cleanupExpected(expected) {',
+    '    function peg$buildException() {',
+    '      function cleanupExpected(expected) {',
     '        var i = 1;',
     '',
     '        expected.sort(function(a, b) {',
@@ -930,6 +931,21 @@ module.exports = function(ast, options) {
     '          }',
     '        }',
     '      }',
+    '',
+    '      var pos        = Math.max(peg$currPos, peg$maxFailPos),',
+    '          posDetails = peg$computePosDetails(pos),',
+    '          found      = pos < input.length ? input.charAt(pos) : null;',
+    '',
+    '      cleanupExpected(peg$maxFailExpected);',
+    '',
+    '      return new SyntaxError(',
+    '        peg$maxFailExpected,',
+    '        found,',
+    '        pos,',
+    '        posDetails.line,',
+    '        posDetails.column',
+    '      );',
+    '    }',
     ''
   ].join('\n'));
 
@@ -959,16 +975,7 @@ module.exports = function(ast, options) {
     '    if (peg$result !== peg$FAILED && peg$currPos === input.length) {',
     '      return peg$result;',
     '    } else {',
-    '      peg$cleanupExpected(peg$maxFailExpected);',
-    '      peg$reportedPos = Math.max(peg$currPos, peg$maxFailPos);',
-    '',
-    '      throw new SyntaxError(',
-    '        peg$maxFailExpected,',
-    '        peg$reportedPos < input.length ? input.charAt(peg$reportedPos) : null,',
-    '        peg$reportedPos,',
-    '        peg$computePosDetails(peg$reportedPos).line,',
-    '        peg$computePosDetails(peg$reportedPos).column',
-    '      );',
+    '      throw peg$buildException();',
     '    }',
     '  }',
     '',

lib/parser.js

@@ -469,7 +469,8 @@ module.exports = (function() {
       peg$maxFailExpected.push(expected);
     }
 
-    function peg$cleanupExpected(expected) {
+    function peg$buildException() {
+      function cleanupExpected(expected) {
         var i = 1;
 
         expected.sort(function(a, b) {
@@ -491,6 +492,21 @@ module.exports = (function() {
         }
       }
 
+      var pos        = Math.max(peg$currPos, peg$maxFailPos),
+          posDetails = peg$computePosDetails(pos),
+          found      = pos < input.length ? input.charAt(pos) : null;
+
+      cleanupExpected(peg$maxFailExpected);
+
+      return new SyntaxError(
+        peg$maxFailExpected,
+        found,
+        pos,
+        posDetails.line,
+        posDetails.column
+      );
+    }
+
     function peg$parsegrammar() {
       var s0, s1, s2, s3, s4;
 
@@ -3109,16 +3125,7 @@ module.exports = (function() {
     if (peg$result !== peg$FAILED && peg$currPos === input.length) {
       return peg$result;
     } else {
-      peg$cleanupExpected(peg$maxFailExpected);
-      peg$reportedPos = Math.max(peg$currPos, peg$maxFailPos);
-
-      throw new SyntaxError(
-        peg$maxFailExpected,
-        peg$reportedPos < input.length ? input.charAt(peg$reportedPos) : null,
-        peg$reportedPos,
-        peg$computePosDetails(peg$reportedPos).line,
-        peg$computePosDetails(peg$reportedPos).column
-      );
+      throw peg$buildException();
     }
   }