Reduce friction for new users

Firstly, thanks heaps for writing Jently. It's super useful, and if I ever run into you in person I shall surely to buy you :beers: or :cake:.

This PR does a few things: 

 - Provide a hint to users if querying the Jenkins API results in a JSON parsing error. This error likely suggests the parameterised job hasn't been set up.
 - Make the log messages a bit easier to read by splitting the backtrace over multiple lines.
 - Use `Pathname` to determine paths to files, instead of `Dir.pwd`. This allows Jently to be invoked from places outside of the directory Jently is cloned into (this is helpful for executing Jently from an init system).
 - Simplify the requiring of libraries by adding `lib/` to the load path.

Please let me know if you have any questions!