ngspice-doc's install-info invocations are not failsafe against excludedocs or
read-only-netsharedpath-/usr/share installations; appending "|| :" to the
invocations in %post and %preun would fix it.
Additionally, install-info works on plain *.info too, no need to explicitly
specify the .gz extension if you like.
More info: http://fedoraproject.org/wiki/Packaging/ScriptletSnippets