tiff: Return GError on TIFF open failure
tiff: Return GError from tiff_read_region()
Return GError from associated image get_argb_data()
hamamatsu: Return GError from jpeg_random_access_src()
hamamatsu: Return GError from compute_mcu_start()
hamamatsu: Return GError from read_from_jpeg()
hamamatsu: Allocate tiledata buffer in read_jpeg_tile()
hamamatsu: Deindent read_from_jpeg() section
hamamatsu: Fix fh leak in background thread error path
hamamatsu: Return GError from restart marker thread
tiff: Return GError from tile-reading functions
aperio: Return GError from decode_tile()
Return GError when checking cairo status
tiff: Return GError from _openslide_tiff_clip_tile()
mirax: Return GError from read_image()
Return GError from read_tile functions
grid: Return GError from _openslide_grid_paint_region()
Return GError from paint_region ops method
Return GError from read_region() helper
Move external error API into separate header

These functions shouldn't be used except by the external API.