Pushing to a non-"public" gives a "no permissions" error instead of suggesting public

[Simon Højberg]

When pushing to a non-"public" namespace on Share Next we should suggest the user to use the "public" namespace instead, explain why, and give an example of a full command based on their previous attempt.

[Arya Irani]

First step: Come up with a good replacement message.

[Mitchell Dalvi Rosen]

<noting that within reason we still want old clients' error messages to make sense after some "projects" work - will public continue to be a thing?>

[Simon Højberg]

After the project work, public will no longer be a thing. I think its worth a dedicated error message in this first release because its so specific.

Here's a suggestion:

Unison Share currently only supports sharing public code. This is done by hosting code in a public namespace under your handle.
It looks like you were trying to push textExtra to the hojberg handle. Try nesting under public like so:
push.create hojberg.public.textExtra .textExtra

If you like, I can just go ahead and do that for you? y/n

The last bit with the prompt would be really awesome, but I understand that we can't do prompts right now in UCM.