lib/repo-commit: Delay propagation of errors from abort_transaction()

lib/repo-commit: Delay propagation of errors from abort_transaction()

If there’s a problem while aborting a transaction, store the error but
don’t report it until the end of the function — do a best effort at
clearing the rest of the transaction state first (since most of it
cannot fail).

If cleanup_tmpdir() fails (which, arguably, should not be a
showstopper), this allows a caller to recover and start a new
transaction in future.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #1626
Approved by: jlebon