Changed failure to create decoder to OSError for Parser

Tests/test_imagefile.py

@@ -82,6 +82,17 @@ def test_ico(self):
             p.feed(data)
             assert (48, 48) == p.image.size
 
+    def test_incremental_webp(self):
+        with ImageFile.Parser() as p:
+            with open("Tests/images/hopper.webp", "rb") as f:
+                p.feed(f.read(1024))
+
+                # Check that insufficient data was given in the first feed
+                assert not p.image
+
+                p.feed(f.read())
+            assert (128, 128) == p.image.size
+
     @skip_unless_feature("zlib")
     def test_safeblock(self):
         im1 = hopper()

src/_webp.c

@@ -395,7 +395,7 @@ _anim_decoder_new(PyObject *self, PyObject *args) {
         }
         PyObject_Del(decp);
     }
-    PyErr_SetString(PyExc_RuntimeError, "could not create decoder object");
+    PyErr_SetString(PyExc_OSError, "could not create decoder object");
     return NULL;
 }