fix `blank` line test for require `headers.size() == 1` +  reduce stderr verbosity.
fix `blank` line test for require `headers.size() == 1` +  reduce stderr verbosity.

tomhughes/mapnik,tomhughes/mapnik,tomhughes/mapnik,lightmare/mapnik,lightmare/mapnik,naturalatlas/mapnik,mapnik/mapnik,naturalatlas/mapnik,naturalatlas/mapnik,mapnik/mapnik,naturalatlas/mapnik,mapnik/mapnik,lightmare/mapnik,tomhughes/mapnik,mapnik/mapnik,lightmare/mapnik