vipsthumbnail sets an error on failure

conversion errors are ignored as before, but on final exit vipsthumbnail will
return an error code if one or more of the conversions failed

see https://github.com/jcupitt/libvips/issues/219

ChangeLog

@@ -8,6 +8,7 @@
 - more tests
 - renamed VIPS_FOREIGN_DZ_DEPTH_1 as VIPS_FOREIGN_DZ_DEPTH_ONE etc. to help
   bindings
+- vipsthumbnail will return an error code if one or more conversions failed
 
 24/12/14 started 7.42.1
 - add gobject-2.0 to Requires: in vips and vips-cpp .pc files

man/vipsthumbnail.1

@@ -120,6 +120,8 @@ normally runs silently, except for warning and error messages. This option
 makes it print a list of the operations it performs on each image.
 
 .SH RETURN VALUE
-returns 0 on success and non-zero on error.
+returns 0 on success and non-zero on error. Error can mean one or more
+conversions failed.
+
 .SH SEE ALSO
 vipsheader(1)

tools/vipsthumbnail.c

@@ -61,6 +61,8 @@
  * 12/9/14
  * 	- try with embedded profile first, if that fails retry with fallback
  * 	  profile
+ * 13/1/15
+ * 	- exit with an error code if one or more conversions failed
  */
 
 #ifdef HAVE_CONFIG_H
@@ -730,6 +732,7 @@ main( int argc, char **argv )
 	GOptionGroup *main_group;
 	GError *error = NULL;
 	int i;
+	int result;
 
 	if( VIPS_INIT( argv[0] ) )
 	        vips_error_exit( "unable to start VIPS" );
@@ -770,6 +773,8 @@ main( int argc, char **argv )
 		thumbnail_height = thumbnail_width;
 	}
 
+	result = 0;
+
 	for( i = 1; i < argc; i++ ) {
 		/* Hang resources for processing this thumbnail off @process.
 		 */
@@ -780,6 +785,11 @@ main( int argc, char **argv )
 				argv[0], argv[i] );
 			fprintf( stderr, "%s", vips_error_buffer() );
 			vips_error_clear();
+
+			/* We had a conversion failure: return an error code
+			 * when we finally exit.
+			 */
+			result = -1;
 		}
 
 		g_object_unref( process );
@@ -787,5 +797,5 @@ main( int argc, char **argv )
 
 	vips_shutdown();
 
-	return( 0 );
+	return( result );
 }