bugtool: Be more resilient to file errors
bugtool: Be more resilient to file errors

When walking paths, if there are any problems while attempting to open
or read the file, skip that file and continue; only fail out on creation
of the archive if there is an error after writing the header, ie
specifically when copying the content of the file into the tar.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

tgraf/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,cilium-team/cilium,cilium/cilium,michi-covalent/cilium,cilium-team/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium