More detailed PrinceXML error messages about magicbook

[shiffman]

I noticed that the error messages can be insanely long if one thing is out of place. I am dealing with this by writing the error to a file to examine, this might be a good option too.

[runemadsen]

Great idea. I'm going to add this today.

[runemadsen]

Okay, this has been fixed in 0.1.4. You can now add the following to your config file:

"pdf" : {
  "log" : "mylog.txt"
}

And each build will have a mylog.txt file with the Prince log.