Better error message for failed list memberships due to private accounts

[dsalo]

Describe

Whenever an attempt to add another account to a list fails, the same error message pops up: "Failed to update the list memberships." When this is due to the account being private and a follow request has not been approved, please have the error message say so.

Alternately, disallow the Manage List Memberships item in the gear menu for private accounts.

Why

This message looks too much like a bug in Whalebird, when it's really a feature of Mastodon's follow system. Explaining to users what's really going on would be a kindness.

[stale]

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