Don't remove pack from pack list for problems which could be transient

Don't remove pack from pack list for problems which could be transient

If we hit a corrupt object or invalid pack remove the pack from the pack
list. Other IOException could be transient hence we should not remove
the pack from the list to avoid the problem reported on the Gerrit list
[1]. It looks like in the reported case the pack was removed from the
pack list causing MissingObjectExceptions which disappear when the
server is restarted.

[1] https://groups.google.com/forum/#!topic/repo-discuss/Qdmbl-YZ4NU

Change-Id: I331626110d54b190e46cddc2c40f29ddeb9613cd
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>