Simplify some ERROR paths clearing wait events and transient files

Transient files and wait events get normally cleaned up when seeing an
exception (be it in the context of a transaction for a backend or
another process like the checkpointer), hence there is little point in
complicating error code paths to do this work.  This shaves a bit of
code, and removes some extra handling with errno which needed to be
preserved during the cleanup steps done.

Reported-by: Masahiko Sawada
Author: Michael Paquier
Reviewed-by: Tom Lane, Masahiko Sawada
Discussion: https://postgr.es/m/CAD21AoDhHYVq5KkXfkaHhmjA-zJYj-e4teiRAJefvXuKJz1tKQ@mail.gmail.com

src/backend/storage/file/copydir.c

@@ -199,7 +199,6 @@ copy_file(char *fromfile, char *tofile)
 		pgstat_report_wait_start(WAIT_EVENT_COPY_FILE_WRITE);
 		if ((int) write(dstfd, buffer, nbytes) != nbytes)
 		{
-			pgstat_report_wait_end();
 			/* if write didn't set errno, assume problem is no disk space */
 			if (errno == 0)
 				errno = ENOSPC;