better (?) error messages

funcs/fimg-png.c

@@ -64,7 +64,8 @@ fprintf(stderr, "%s opened\n", filename);
 datasize = png.width * png.height * png.bpp;
 
 if ( 3 != png.bpp ) {
-	fprintf(stderr, "format %d of '%s' not supported\n",
+	/* I don't really understand this part of the code */
+	fprintf(stderr, "bpp format %d of '%s' not supported\n",
 			png.color_type, filename);
 	return -21;
 	}
@@ -93,7 +94,7 @@ if (foo) {
 
 foo = png_get_data(&png, datas);
 if (PNG_NO_ERROR != foo) {
-	fprintf(stderr, "error in '%s' : read png -> %d %s\n",
+	fprintf(stderr, "error in '%s' :\n\tpng_get_data -> %d = %s\n",
 			__func__, foo, pngerr2str(foo));
 	return foo;
 	}