Build PDF failed

I try to build with
```
magicbook build
```

It print out console like this
```
Config file detected: magicbook.json
Prince XML error
-> HTML build finished
```

In build folder, there are no PDF(s)

Start by running `magicbook build` and compare the console output with the contents of the build folder. Investigate the reported Prince XML error and the HTML-to-PDF build step. Done means the command completes with a PDF present in the build folder.

Written by the indexing model from the issue text.