Incorrect Dancer2::Core::Error rendering

Exceptions should not appear after the rendered error page when
show_errors is true

Currently, if show_errors is true, and a die appears in a route, the default dancer app will render an html error page with the error message after the &lt;/html&gt; tag. Which is of course invalid. Similarly if the error is serialized in some other way, the exception gets tacked onto the end incorrectly. This is a result of 58907995f1350427d18452e5c03bb2ff7c579417
Issue #216

This PR only fixes that, however, there are a bunch of other things that are broken in Dancer2::Core::Error that I started to fix before realising that it would complicate a merge, such as stacktraces are ignored, config documentation says there is a setting called traces, but this doesn't affect the use of the backtrace function in D2:C:E
We'll need to check if $value is defined, otherwise perl false values (like '0') will become undefs.