Master exceptions b sig

Using UserError, AccessError, MissingError, ValidationError instead of except_orm and except_osv.
Using _logger.info instead of _logger.warning to avoid polluating Sentry

To better show that ``Warning`` is now deprecated, we could just keep it as an alias of `UserError`, i.e.:

```python
class UserError(except_orm):
    pass

# deprecated due to collision with builtins, kept for compatibility
Warning = UserError
```