When a filesystem does't support xattr return a 507

Currently when the object server tries to write an object's metadata
to a filesystem that doesn't support xattr, it errors with a stacktrace
and returns a 500 error back to the user with no information.

This patch catches the resulting IOError when attempting to read or write
the xattr metadata, logs the error nicely and then returns a 507 error
back to the user.

Seeing as this change is sending back a 507, it also catches and logs
the out of disk space errors (ENOSPC and EDQUOT).

Change-Id: I31932b57582817a0b3b58dd315a996bd0bcbc99b
Closes-Bug: #966671
Merge "When a filesystem does't support xattr return a 507"