lilypond'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, if you like, there's no need to explicitly specify the .gz extensions, install-info works with plain *.info. More info: http://fedoraproject.org/wiki/Packaging/ScriptletSnippets
This is fixed in the 2.10.2-2 builds.
Actually, the removals in %preun should be wrapped inside "if [ $1 -eq 0 ] ; then remove ; fi" - otherwise the info pages will be removed from the index on package upgrades. See the scriptletsnippets page from initial comment for more info.
OK, I'm adding that in the 2.10.4-1 builds.