Document errors

[peterpeterparker]

Errors are now prefixed - e.g. export const JUNO_ERROR_NO_TIMESTAMP_PROVIDED = 'juno.error.no_timestamp_provided'; - and also exposed in a library @junobuild/errors.

So we should:

• Comment the errors in the Rust code - i.e. for each "JUNO_..." add a comment to explain when and why it can happens
• Generate a script that copy over the defined constants in Rust to the library @junobuild/errors
• Make sure the documentation is exposed
• Finally, add a documentation page on the website that also automatically inherits the list of errors, a bit like updating the CLI information