Fix some error cases in the caching layer. (#5749)

There was some inconsistent behaviour in the caching layer around how
exceptions were handled - particularly synchronously-thrown ones.

This seems to be most easily handled by pushing the creation of
ObservableDeferreds down from CacheDescriptor to the Cache.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/5749.misc b/changelog.d/5749.misc
new file mode 100644
index 0000000000..48dd61f461
--- /dev/null
+++ b/changelog.d/5749.misc
@@ -0,0 +1 @@
+Fix some error cases in the caching layer.