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

@@ -909,26 +909,42 @@ module.exports = function(ast, options) {
     '      peg$maxFailExpected.push(expected);',
     '    }',
     '',
-    '    function peg$cleanupExpected(expected) {',
-    '      var i = 1;',
-    '',
-    '      expected.sort(function(a, b) {',
-    '        if (a.description < b.description) {',
-    '          return -1;',
-    '        } else if (a.description > b.description) {',
-    '          return 1;',
-    '        } else {',
-    '          return 0;',
-    '        }',
-    '      });',
+    '    function peg$buildException() {',
+    '      function cleanupExpected(expected) {',
+    '        var i = 1;',
+    '',
+    '        expected.sort(function(a, b) {',
+    '          if (a.description < b.description) {',
+    '            return -1;',
+    '          } else if (a.description > b.description) {',
+    '            return 1;',
+    '          } else {',
+    '            return 0;',
+    '          }',
+    '        });',
     '',
-    '      while (i < expected.length) {',
-    '        if (expected[i - 1].description === expected[i].description) {',
-    '          expected.splice(i, 1);',
-    '        } else {',
-    '          i++;',
+    '        while (i < expected.length) {',
+    '          if (expected[i - 1].description === expected[i].description) {',
+    '            expected.splice(i, 1);',
+    '          } else {',
+    '            i++;',
+    '          }',
     '        }',
     '      }',
+    '',
+    '      var pos        = Math.max(peg$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();',
     '    }',
     '  }',
     '',