Only report the first class in obj_type_friendly()

[DavisVaughan]

Motivated by the fact that subclassing things like rcrds can result in very distracting class lists, with no added benefit

> zoned_time_zone(duration_days(1))
Error in `zoned_time_zone()`:
! `x` must be a <clock_zoned_time>, not a <clock_duration/clock_rcrd/vctrs_rcrd/vctrs_vctr> object.