Improve error message when invalid Entity List name is given

[lognaturel]

@n-orlowski tried to create an Entity List and included a space in its name. The error came from the backend and was not helpful:

https://github.com/getodk/central-backend/blob/4af4f33c6dd5c8f1c76a0222e143bf2d74e9e5da/lib/resources/datasets.js#L61

Let's add a frontend error message that provides more information on what a valid list name is.

Consider:

• Adding guidance on what a valid name is before the user specifies it.
• Describing that the list name is an id, that it will be used in form design on the entities tab