Skip importing AwardEmoji for custom emoji which don't exist on the target instance

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When using Bulk Imports to import a project or group, if the source has custom emoji, which the destination does not have, we'll get a RecordInvalid error with "Name is not a valid emoji name" when trying to create the AwardEmoji.

This error isn't very valuable to the end user so we should instead skip creating these records in the first place, and update the documentation to make it clear that missing custom emoji will not be imported.

Related to: https://gitlab.com/gitlab-com/request-for-help/-/issues/2741#note_2473589482