pegjs: "unexpected" rule

• Feature Request:

Add “unexpected” rule to override standard error message.

unexpected = 
      keywords { return "Unexpected keyword "+text()+"."; }
    / expression { return "Unexpected expression «"+text()+"»."; }
    / lamdba_function { return "Unexpected lambda function."; } ;

To implement it just change peg$buildError by:

  function peg$buildError() {
    var expected = peg$expected[0];
    var failPos = expected.pos;

    // if "unexpected" rule exist this might throw the appropriate error.
    if (typeof peg$parseunexpected !== 'undefined') {
      // make peg$expect temporary unavailable, set the cursor position to the fail position
      // next get the output of the rule, if it's a string, return it.
      const tmp = peg$expect;
      peg$expect = new Function();
      peg$currPos = failPos;
      const unexpected = peg$parseunexpected();
      peg$expect = tmp;
      if (typeof unexpected === 'string') {
        const length = peg$currPos - failPos;
        const location = failPos < input.length
        ? peg$computeLocation(failPos, failPos + length)
        : peg$computeLocation(failPos, failPos);
        return new peg$SyntaxError(unexpected, expected.variants, unexpected, location);
      }
    }
    // else return standard error.
    const unexpected = input.charAt(failPos);
    const location = failPos < input.length
        ? peg$computeLocation(failPos, failPos + 1)
        : peg$computeLocation(failPos, failPos);
    return new peg$SyntaxError(
      peg$SyntaxError.buildMessage(expected.variants, unexpected), expected.variants, unexpected, location);
  }

Expected behavior: Improve error handling.

If it’s not ethical, is there anyone who can tell me how to create a plugin that would make the change? Thanks a lot