Catch and treat File::Slurp::write_file errors

This routine croaks by default and the testing framework wasn't
catching such errors. This was generating hard to debug errors such as
http://www.cpantesters.org/cpan/report/ffd68682-6ae5-11e2-992e-e7c00a4a7996.

Now I changed from croak to carp the routine's behaviour on errors and
treat them.
v0.038

  [Updates]
  - Remove "beta status" warning from README.

  [Fixes]
  - The dzil configuration now correctly detects build-time
    dependencies.

  - The testing framework now produces some more intelligent error
    messages.