Making btoa polyfill more standard

lib/helpers/btoa.js

@@ -8,6 +8,7 @@ function InvalidCharacterError(message) {
   this.message = message;
 }
 InvalidCharacterError.prototype = new Error;
+InvalidCharacterError.prototype.code = 5;
 InvalidCharacterError.prototype.name = 'InvalidCharacterError';
 
 function btoa (input) {
@@ -24,11 +25,11 @@ function btoa (input) {
   ) {
     charCode = str.charCodeAt(idx += 3/4);
     if (charCode > 0xFF) {
-      throw new InvalidCharacterError('\'btoa\' failed: The string to be encoded contains characters outside of the Latin1 range.');
+      throw new InvalidCharacterError('INVALID_CHARACTER_ERR: DOM Exception 5');
     }
     block = block << 8 | charCode;
   }
   return output;
 };
 
-module.exports = btoa
+module.exports = btoa;