rutil: add FileSystem::Exception, check stat() return value

git-svn-id: https://svn.resiprocate.org/rep/resiprocate/main@10371 ddefafc4-47db-0310-ae44-fa13212b10f2
rutil: FileSystem: check results of readdir calls

git-svn-id: https://svn.resiprocate.org/rep/resiprocate/main@10372 ddefafc4-47db-0310-ae44-fa13212b10f2