Check that a channel exists locally before trying to import content

[rtibbles]

Summary

• Adds some simple validation to the importchannel command

References

Fixes #7506

Reviewer guidance

Does importcontent still work as expected?

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[jredrejo]

LGTM, it's totally harmless. I'd maybe refactor it to assign options["channel_id"] to a variable to avoid several dict lookups in the same function