Export errorHandler method

Exporting the errorHandler method as public API allows handling of errors in async code.

Example:
var swagger    = require("swagger-node-express");

exports.remove = {
    'spec' : {
      "description" : "Deletes an object",
      "path" : "/objects/{id}",
      "notes" : "",
      "summary" : "",
      "method" : "DELETE",
      "params" : [swagger.pathParam("id", "ID of object to delete", "string")],
      "errorResponses" : [ swagger.errors.notFound('id'), swagger.errors.invalid('id') ],
      "nickname" : "remove"
    },
    'action' : function(req, res) {

      if (!req.params.id) {
        throw swagger.errors.invalid('id');
      }

      req.coll.removeById(req.params.id, function(err, result) {
        if (err) {
          swagger.errorHandler(req, res, swagger.errors.invalid('collection'));
        }

        res.send({msg : 'success'});
      });
    }
  };
Merge pull request #77 from pascal-bach/master

Export errorHandler method
add some simple messages to build in error functions