Handle cases when authorized_keys doesn't exist

What does this MR do?

Modify Gitlab::AuthorizedKeys#rm_key and #list_key_ids to handle cases when an Errno::ENOENT is raised.

Also added test cases in spec to ensure each method will still work when authorized_keys doesn't exist.

What are the relevant issue numbers?

#10514 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #10514 (closed)