die with a useful error message when the Changes file cannot be found

remedy for:

16:11 < BooK> Can't call method "encoding" on an undefined value at /home/book/perl5/lib/perl5/Dist/Zilla/Plugin/NextRelease.pm line 121.
16:11 < BooK> not sure what the cause is
16:21 < ether> BooK: it's trying to find the 'Changes' file in the dist so it can change {{$NEXT}} to the timestamp, but no such file was found.