Support EDITOR having arguments in encrypted secrets Rake tasks

What does this MR do and why?

Previously if you had EDITOR set to something like emacs -nw, rake gitlab:ldap:secret:edit would quietly fail without launching an editor and write a blank encrypted secrets file to disk. This commit does two things to improve this:

1. Splits the EDITOR environment variable and passes the arguments with a splat operator to system to handle multiple arguments.
2. Checks the return value of the system to ensure that the editor returned successfully.

Relates to #431339 (closed)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. In your gitlab dir, mkdir -p shared/encrypted_settings.
2. Run: GITLAB_GENERATE_ENCRYPTED_SETTINGS_KEY_BASE=1 EDITOR='emacs -nw' bundle exec rake gitlab:ldap:secret:edit
3. Verify the editor comes up.

MR acceptance checklist

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