docs, build: Do not ignore install-docs errors

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>