Return sensible errors when semicolons are detected

[jarjee]

This is a pretty common mistake to make, and it's never valid to have a semicolon in ejson.

We can trap the case where a semicolon is found (in the wrong location), and then return a more sensible error message.

IE it could look something like this, if we can't get the numbers out more easily.

      const match = /Unexpected token \(([0-9]+):([0-9]+)\)/.exec(e.message);
      const line = Number(match[1]) - 1;
      const column = Number(match[2]) - 1;
      const char = input.split('\n')[line][column];

      if (char === ';') {
        throw new Error(`Unexpected semicolon at (${match[1]}:${match[2]})`);
      }