validateMultiple throws exception on multiple errors

[fbrunet99]

validateMutliple may throw an exception when there are multiple errors. It tries to access "this.error.message" but this.error is undefined. Instead, "this.errors" is an array that contains the errors.

A possible fix is to change tv4.js line 1603-1605 to this so it can handle either this.error or this.errors:

var result = {toString: function () {
  return this.valid ? 'valid' : this.error ? this.error.message : this.errors ? JSON.stringify(this.errors, null, '\t') : 'invalid'
}};