doc-rst: Break shell command sequences on failure

doc-rst: Break shell command sequences on failure

As $(SHELL) doesn't include the -e option, any loop or other sequence
needs to include explicit checks for failing commands.

Fixes: 5ca8a91b013e ("Documentation/sphinx: build the media intermediate ...")
Fixes: e1b6a28be8e2 ("doc-rst: generic way to build only sphinx sub-folders")
Fixes: bf827f63b1b9 ("doc-rst: generic way to build PDF of sub-folders")
Tested-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>