Error handling

Exploration of error handling, inspired by vlang, using error recovery operator !?.

-- function returning an error
get-user: (id: #number) -> #user!  -- function typed as returning either a #user or an #error
    db.get(table: 'user', id) ? error "User { id } not found"
    -- returns an #error if db.get() returns #nothing

-- default value
user: get-user(7) !? default-user

-- propagate error (early return)
user: get-user(7) !? return error('Loading failed', [cause: _])

-- pattern matching of error
user: get-user(7) !? match _
    #db-error(err) -> return err
    _ -> default-user