"Update failed" message when checking a box in an epic description

I realize we don't support todo lists in epic descriptions, but we should probably not display an error message when checking a box created in markdown:

/cc @ClemMakesApps

related: #3254 (closed)

Design and specifications

• The markdown still renders checkboxes as normal.
• You can click on them as usual. But there's no special todo list integration.
• When you check or uncheck a checkbox, it saves it to the system, per https://gitlab.com/gitlab-org/gitlab-ee/issues/4021#note_47223218.
• There should not be an error message.