docs, build: Do not ignore install-docs errors

In the toplevel Makefile "install-docs" (depended on by "install" and
hence "dist"), but not "build", ignores errors.

This was inherited from before 24563:4271634e4c86, prior to which the
||true seems intended to handle failures of check_pkgs.  Nowadays we
handle docs tools individually in the docs makefiles so there is no
need for this ||true here.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

Makefile

@@ -103,7 +103,7 @@ tools/firmware/seabios-dir-force-update:
 
 .PHONY: install-docs
 install-docs:
-	$(MAKE) -C docs install || true
+	$(MAKE) -C docs install
 
 .PHONY: dev-docs
 dev-docs: