Replace pgf image-streaming warning by error.

Saving a pgf file with images to an in-memory buffer
(`import io; imshow([[1, 2]]); savefig(io.BytesIO(), format="pgf")`)
currently warns that this is not possible (because the image needs to be
written to the filesyste), but in fact an exception gets raised
immediately after when trying to construct `Path(self.fh.name)`, so we
may as well replace the original warning by an exception.

Author: anntzer

lib/matplotlib/backends/backend_pgf.py

@@ -624,9 +624,9 @@ def draw_image(self, gc, x, y, im, transform=None):
             return
 
         if not os.path.exists(getattr(self.fh, "name", "")):
-            _api.warn_external(
+            raise ValueError(
                 "streamed pgf-code does not support raster graphics, consider "
-                "using the pgf-to-pdf option.")
+                "using the pgf-to-pdf option")
 
         # save the images to png files
         path = pathlib.Path(self.fh.name)