src: bail out when exporting ruleset with unsupported output

src: bail out when exporting ruleset with unsupported output

Display error message and propagate error to shell when running command
with unsupported output:

 # nft export ruleset json
 Error: this output type is not supported
 export ruleset json
 ^^^^^^^^^^^^^^^^^^^^
 # echo $?
 1

When displaying the output in json using the low-level VM
representation, it shows:

 # nft export ruleset vm json
 ... low-level VM json output
 # echo $?
 0

While at it, do the same with obsoleted XML output.

Fixes: https://bugzilla.netfilter.org/show_bug.cgi?id=1224
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>