Add built-in ERROR handling for archive callbacks.

Presently, the archiver process restarts when an archive callback
ERRORs.  To avoid this, archive module authors can use sigsetjmp(),
manage a memory context, etc., but that requires a lot of extra
code that will likely look roughly the same between modules.  This
commit adds basic archive callback ERROR handling to pgarch.c so
that module authors won't ordinarily need to worry about this.
While this built-in handler attempts to clean up anything that an
archive module could conceivably have left behind, it is possible
that some modules are doing unexpected things that require
additional cleanup.  Module authors should be sure to do any extra
required cleanup in a PG_CATCH block within the archiving callback.

The archiving callback is now called in a short-lived memory
context that the archiver process resets between invocations.  If a
module requires longer-lived storage, it must maintain its own
memory context.

Thanks to these changes, the basic_archive module can be greatly
simplified.

Suggested-by: Andres Freund
Reviewed-by: Andres Freund, Yong Li
Discussion: https://postgr.es/m/20230217215624.GA3131134%40nathanxps13

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