more improvements to error handling during eval

we were dropping the error buffer in tilecache, thanks David

TODO

@@ -1,3 +1,41 @@
+- try:
+
+	$ vips avg broken.jpg[fail]
+
+  about 50% of the time it'll trigger a range of out-of-order reads and lock
+  for 10s or so while seq waits
+
+- try:
+
+	#!/usr/bin/python
+
+	import sys
+
+	import gi
+	gi.require_version('Vips', '8.0')
+	from gi.repository import Vips
+
+	try:
+	    im = Vips.Image.new_from_file(sys.argv[1])
+	    im.avg()
+	except Vips.Error as e:
+	    print 'saw an error <<%s>>' % e.message
+	    print 'vips error buffer start:'
+	    print Vips.error_buffer()
+	    print 'vips error buffer stop'
+
+  see:
+
+	$ ./read.py broken.jpg[fail]
+	vips warning: linecache: error in tile 0 x 192
+	vips warning: linecache: error in tile 0 x 200
+	saw an error <<Error calling operator avg.>>
+	vips error buffer start:
+
+	vips error buffer stop
+
+  error msg is lost
+
 - add more webp tests to py suite
 
 - try moving some more of the CLI tests to py

libvips/conversion/tilecache.c

@@ -703,11 +703,8 @@ vips_tile_cache_gen( VipsRegion *or,
 						"error on tile %p\n", tile ); 
 
 					vips_warn( class->nickname,
-						_( "error reading tile %dx%d: "
-							"%s" ),
-						tile->pos.left, tile->pos.top,
-						vips_error_buffer() ); 
-					vips_error_clear();
+						_( "error in tile %d x %d" ),
+						tile->pos.left, tile->pos.top );
 
 					vips_region_black( tile->region );
 

libvips/foreign/jpeg2vips.c

@@ -977,27 +977,29 @@ read_jpeg_generate( VipsRegion *or,
 	 * a vips_sequential().
 	 */
 	if( r->top != jpeg->y_pos ) {
+		VIPS_GATE_STOP( "read_jpeg_generate: work" );
 		vips_error( "VipsJpeg", 
 			_( "out of order read at line %d" ), jpeg->y_pos );
+
 		return( -1 );
 	}
 
 	/* Here for longjmp() from vips__new_error_exit().
 	 */
-	if( setjmp( jpeg->eman.jmp ) ) 
+	if( setjmp( jpeg->eman.jmp ) ) {
+		VIPS_GATE_STOP( "read_jpeg_generate: work" );
+
 		return( -1 );
+	}
 
 	/* If --fail is set, we make read fail on any warnings. This will stop
 	 * on any errors from the previous jpeg_read_scanlines().
 	 */
 	if( jpeg->eman.pub.num_warnings > 0 &&
 		jpeg->fail ) {
-		vips_error( "VipsJpeg", 
-			_( "read gave %ld warnings" ), 
-			jpeg->eman.pub.num_warnings );
-		vips_error( NULL, "%s", vips_error_buffer() );
+		VIPS_GATE_STOP( "read_jpeg_generate: work" );
 
-		/* Make the message only appear once.
+		/* Only fail once.
 		 */
 		jpeg->eman.pub.num_warnings = 0;