tests: Handle missing reference files
tests: Handle missing reference files

Resolve a small bug in our reference PDF to generated PDF comparison,
whereby a missing reference PDF will generate the following failure:

  Mismatch between type of output (file) and reference (dir)

This will now return the following:

  No reference file at 'rst2pdf/tests/reference/foo' to compare against

Signed-off-by: Stephen Finucane <06fa905d7f2aaced6dc72e9511c71a2a51e8aead@that.guru>

rst2pdf/rst2pdf,rst2pdf/rst2pdf