Improve unchop() error for incompatible ptype

[mgirlich]

Fixes #1477

[DavisVaughan]

Out of curiosity I did a little research into why we didn't already have this in place. I think its because:

#1340 is when we added error_call to df_unchop() in October 2022

But list_unchop() didn't get error_call and error_arg until around the same time, Sept 2022 and Oct 2022 respectively
r-lib/vctrs#1691
r-lib/vctrs#1712

So those args probably didn't exist in list_unchop() at the time, but they do now!

[DavisVaughan]

Thanks @mgirlich!