Fix Dancer2::Core::Error

Exceptions should not appear after the rendered error page when
show_errors is true
Dancer2 supports perl 5.8+

Should not rely on 5.10+ keywords such as //
Don't coerce false into undef

As @veryrusty suggests, we should be checking that $value is defined
If we check that it is true, then we turn 0 and '' into undef

Cleanup some of the 'or' ops now that message has a defualt
Reflect changes
Merge branch 'shumphrey-fix-dancer2-core-error'