When using `docker rm` / `docker container rm` with the `-f` / `--force` option, attempts to remove non-existing containers should print a warning, but should return a zero exit code ("successful").

Currently, a non-zero exit code is returned, marking the removal as "failed";

	$ docker rm -fv 798c9471b695
	Error: No such container: 798c9471b695
	$ echo $?
	1

The command should match the behavior of `rm` / `rm -f`, with the exception that
a warning is printed (instead of silently ignored):

Running `rm` with `-f` silences output and returns a zero exit code:

    touch some-file && rm -f no-such-file some-file; echo exit code: $?; ls -la
    # exit code: 0
    # total 0
    # drwxr-xr-x    2 sebastiaan  staff    64 Aug 14 12:17 .
    # drwxr-xr-x  199 sebastiaan  staff  6368 Aug 14 12:13 ..

    mkdir some-directory && rm -rf no-such-directory some-directory; echo exit code: $?; ls -la
    # exit code: 0
    # total 0
    # drwxr-xr-x    2 sebastiaan  staff    64 Aug 14 12:17 .
    # drwxr-xr-x  199 sebastiaan  staff  6368 Aug 14 12:13 ..

Note that other reasons for a delete to fail should still result in a non-zero
exit code, matching the behavior of `rm`. For instance, in the example below,
the `rm` failed because directories can only be removed if the `-r` option is used;

    touch some-file && mkdir some-directory && rm -f some-directory no-such-file some-file; echo exit code: $?; ls -la
    # rm: some-directory: is a directory
    # exit code: 1
    # total 0
    # drwxr-xr-x    3 sebastiaan  staff    96 Aug 14 14:15 .
    # drwxr-xr-x  199 sebastiaan  staff  6368 Aug 14 12:13 ..
    # drwxr-xr-x    2 sebastiaan  staff    64 Aug 14 14:15 some-directory

This patch updates the `docker rm` / `docker container rm` command to not produce
an error when attempting to remove a missing containers, and instead only print
the error, but return a zero (0) exit code.

With this patch applied:

    docker create --name mycontainer busybox \
    && docker rm nosuchcontainer mycontainer; \
    echo exit code: $?; \
    docker ps -a --filter name=mycontainer
    # df23cc8573f00e97d6e948b48d9ea7d75ce3b4faaab4fe1d3458d3bfa451f39d
    # mycontainer
    # Error: No such container: nosuchcontainer
    # exit code: 0
    # CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>