CI: Fail silently if can't get prev screenshots

...to compare to. Not being able to fetch previous screenshots, or not
being to take current screenshots for that matter, is not a problem with
the code and shouldn't be a test failure, I feel like.

This has happened a whole bunch, here's 10 recent examples:
https://circleci.com/gh/mathquill/mathquill/304
https://circleci.com/gh/mathquill/mathquill/298
https://circleci.com/gh/mathquill/mathquill/296
https://circleci.com/gh/mathquill/mathquill/293
https://circleci.com/gh/mathquill/mathquill/272
https://circleci.com/gh/mathquill/mathquill/270
https://circleci.com/gh/mathquill/mathquill/269
https://circleci.com/gh/mathquill/mathquill/267
https://circleci.com/gh/mathquill/mathquill/266
https://circleci.com/gh/mathquill/mathquill/265
diff --git a/circle.yml b/circle.yml
index 922d880..41f322d 100644
--- a/circle.yml
+++ b/circle.yml
@@ -185,11 +185,14 @@
 
     # Download the latest mathquill artifacts.
     - |-
-      curl $(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts \
-              | $json -a url pretty_path -d '\n\t' \
-              | grep '\.png$' \
-              | grep -v 'PREV_' \
-              | sed "s:\$CIRCLE_ARTIFACTS/imgs/:-o $CIRCLE_ARTIFACTS/imgs/PREV_:")
+      prev_imgs="$(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts \
+                    | tee /dev/stderr
+                    | $json -a url pretty_path -d '\n\t' \
+                    | grep '\.png$' \
+                    | grep -v 'PREV_' \
+                    | sed "s:\$CIRCLE_ARTIFACTS/imgs/:-o $CIRCLE_ARTIFACTS/imgs/PREV_:"
+                    | tee /dev/stderr)"
+      test -z "$prev_imgs" || curl $prev_imgs
 
     # Generate image diffs.
     - |-