10.0.0: basic_format_parse_context lost on_error API

[Dani-Hub]

Starting with fmtlib 10, the following code does no longer compile anymore:

https://godbolt.org/z/W8oT9WG1q

This code is the verbatim example from

https://fmt.dev/10.0.0/api.html#formatting-user-defined-types

which uses the on_error member function from basic_format_parse_context to handle errors. Similar code which is part of our company's code base broke due to that change.

Initially I assumed this change was intentional (and that the documented API example should be adjusted), but since the previous on_error delegated to internal API (Such as throw_format_error) it is unclear how use code should correctly handle the error without referring to internal API.

[vitaut]

Duplicate of #3458.