Reduce log level of cupsRasterOpen failure

If cupsRasterOpen fails, the most likely cause is that some earlier
filter didn't produce any output. If we output an error-level message
in this situation, we'll likely displace some earlier, more useful
message by the other filter.

src/main.cc

@@ -158,7 +158,7 @@ int main(int argc, char *argv[]) {
 
   ras = cupsRasterOpen(fd, CUPS_RASTER_READ);
   if (!ras) {
-    fprintf(stderr, "ERROR: " PACKAGE ": Can't read raster data\n");
+    fprintf(stderr, "DEBUG: " PACKAGE ": Cannot read raster data. Most likely an earlier filter in the pipeline failed.\n");
     return 1;
   }