Fix two possible UnicodeDecodeError when an exception occurs

# Description

Using the `%` operator with strs containing encoded unicode characters and a unicode string will cause ascii errors. the `ustr()` function guards against this.

In the error handling of http.py, there are two instances of the use of the `%` operator with potential Ascii errors.

# How to reproduce:

Raise an error in a 8.0 create function with an ascii str with unicode characters such as 
```
    @api.model
    def create(self, vals):
        u'%s' % '\xc3\xa9'
```