coredumpctl: delay the "on tty" refusal until as late as possible

coredumpctl: delay the "on tty" refusal until as late as possible

For the user, if the core file is missing or inaccessible, it is
more interesting that the fact that they forgot to pipe to a file.
So delay the failure from the check until after we have verified
that the file or the COREDUMP field are present.

Partially fixes #4161.

Also, error reporting on failure was duplicated. save_core() now
always prints an error message (because it knows the paths involved,
so can the most useful message), and the callers don't have to.