import: try to capture tar exit codes on failure

import: try to capture tar exit codes on failure

TEST-13-NSPAWN.machined occasionally fails with a tar error, and it's hard
to say what the problem is at the exit code is lost. Try to capture it.

[ 34.054] systemd-importd[504]: (transfer18) Imported 92%.
[ 34.118] systemd-importd[504]: (transfer18) Failed to decode and write: Broken pipe
[ 34.119] systemd-importd[504]: (transfer18) Exiting.
[ 34.121] systemd-importd[504]: (transfer18) Failed to allocate transient user namespace: Operation not permitted
[ 34.121] systemd-importd[504]: Transfer process failed with exit code 1.

Follow-up for b6e676ce41508e2aeea22202fc8f234126177f52