btrfs: unfold transaction aborts when replaying log trees

We have a single line doing a transaction abort in case either we got an
error from btrfs_get_fs_root() different from -ENOENT or we got an error
from btrfs_pin_extent_for_log_replay(), making it hard to figure out which
function call failed when looking at a transaction abort massages and
stack trace in dmesg. Change this to have an explicit transaction abort
for each one of the two cases.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>