Pr/216

https://github.com/PerlDancer/Dancer2/issues/216

I did add the show_errors. I tried to reconstruct what was meant with the show_errors attribute. Seems not exactly perfect. I had to add new show_errors attribute to tests. I also added some new simple/basic error tests. Not sure we need them really.

Core::Error is still a bit messy. I didn't solve the issues with template,

Also teach the Core::Role::Logger to be polite and to say 'undef' when message is undefined (like D1).

Sorry, for bad commit messages today!