Improve the error messages
In a lot of cases, when the dataset viewer has an error, the error message is not clear at all, or exposes internals of the project which are not important for the user, etc.

We should aim at providing information tailored for the Hub user: 
- the error comes from the repo: what can they do to fix the error?
- the error comes from the server:
  - it's normal, just wait ... minutes before trying again
  - it's not normal, report here: ...

Improve the error messages: In a lot of cases, when the dataset viewer has an error, the error message is not clear at all, or exposes internals of the project which are not important for the user, etc.

We should aim at providing information tailored for the Hub user: 
- the error comes from the repo: what can they do to fix the error?
- the error comes from the server:
  - it's normal, just wait ... minutes before trying again
  - it's not normal, report here: ...
