Name conflicts with existing system emoji name

[kc9jud]

If you have a file that conflicts with a builtin emoji, you'll get an error:

Error: Name conflicts with existing system emoji name.

It seems like this should be handled gracefully (i.e. skipped over), especially in --no-clobber mode.

[kc9jud]

In particular, this shows up if you try to import the party parrots (cultofthepartyparrot.com) since there's a file named parrot.gif which conflicts with Mattermost's builtin :parrot: