[puppetfile] Wrap SyntaxError and LoadError on load

SyntaxError and LoadError don't inherit from StandardError, so if
the process of loading a Puppetfile raises either one of those, it will
completely unwind the stack and crash the r10k process. Since this is a
special case where we're loading Ruby code that's not actual application
code we need to rescue and wrap these errors.
Merge pull request #220 from adrienthebo/defect/master/gh-214-puppetfile-syntax-error

[puppetfile] Wrap SyntaxError and LoadError on load