Make cast functions from jsonb error safe

Make cast functions from jsonb error safe

This adjusts cast functions from jsonb to other types to support soft
errors.  This just involves some refactoring of the underlying helper
functions to use ereturn.

This is in preparation for a future feature where conversion errors in
casts can be caught.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: Amul Sul <sulamul@gmail.com>
Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CADkLM%3Dfv1JfY4Ufa-jcwwNbjQixNViskQ8jZu3Tz_p656i_4hQ%40mail.gmail.com