btrfs: handle invalid range and start in merge_extent_mapping()

Turn a BUG_ON to a properly handled error and update the error message
in the caller.  It is expected that @em_in and @start passed to
btrfs_add_extent_mapping() overlap. Besides tests, the only caller
btrfs_get_extent() makes sure this is true.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>