Allow wand to fail if it doesn't have the right libs

If it can't do it, defer to pillow. This shouldn't really be
needed as if one can't do it then the other shouldn't be able
to as they probably use the same libs under the hood, but hey
this seems to work...

Also, wand is much faster at converting tifs than pillow, which
is why this code exists in the first place.

iiif/utils.py

@@ -16,6 +16,7 @@
 from itertools import count
 from jpegtran import JPEGImage
 from lru import LRU
+from wand.exceptions import MissingDelegateError
 from wand.image import Image as WandImage
 
 
@@ -192,22 +193,33 @@ def convert_image(image_path: Path, target_path: Path, quality: int = 80,
     :param quality: the jpeg quality setting to use
     :param subsampling: the jpeg subsampling to use
     """
-    with WandImage(filename=str(image_path)) as image:
-        if image.format.lower() == 'jpeg':
-            # if it's a jpeg, remove the orientation exif tag. We do this because through trial and
-            # error it seems the dimensions provided by EMu are non-orientated and therefore we need
-            # to work on the images without their orientation too and serve them up without it
-            # otherwise things start to go awry
-            image.strip()
-        else:
-            image.format = 'jpeg'
-
-        image.compression_quality = quality
-        image.options['jpeg:sampling-factor'] = subsampling
-
-        target_path.parent.mkdir(parents=True, exist_ok=True)
-        with target_path.open('wb') as f:
-            image.save(file=f)
+    try:
+        with WandImage(filename=str(image_path)) as image:
+            if image.format.lower() == 'jpeg':
+                # if it's a jpeg, remove the orientation exif tag. We do this because through trial
+                # and error it seems the dimensions provided by EMu are non-orientated and therefore
+                # we need to work on the images without their orientation too and serve them up
+                # without it otherwise things start to go awry
+                image.strip()
+            else:
+                image.format = 'jpeg'
+
+            image.compression_quality = quality
+            image.options['jpeg:sampling-factor'] = subsampling
+
+            target_path.parent.mkdir(parents=True, exist_ok=True)
+            with target_path.open('wb') as f:
+                image.save(file=f)
+    except MissingDelegateError:
+        with Image.open(image_path) as image:
+            if image.format.lower() == 'jpeg':
+                exif = image.getexif()
+                # this is the orientation tag, remove it if it's there
+                exif.pop(0x0112, None)
+                image.info['exif'] = exif.tobytes()
+
+            target_path.parent.mkdir(parents=True, exist_ok=True)
+            image.save(target_path, format='jpeg', quality=quality, subsampling=subsampling)
 
 
 def create_logger(name: str, level: Union[int, str]) -> logging.Logger: