Rewrite API errors

Errors sent to the front are Meh tier. For instance:

It's ugly, its not user friendly...

This is a huge rework:

• API should return error codes

• Error codes should be interpreted by front app to translate errors correctly

  • in {locale}.json just add a new prefix "error_" or something like that
  • error string should be user friendly from now on.
• We should enforce Claude to send always send an error code, never a raw string.