Provide restarts in case of string decoding errors.

Clean unattended processing of encoding errors (as easily enough found in MySQL databases) require new restart cases to be provided. This patch has been tested with pgloader for non-interactive call of the restarts, and interactively like this:

```lisp
(let ((babel::*default-character-encoding* :utf-8))
           (qmynd-impl::decode-octets-to-string (coerce #(209) '(vector (unsigned-byte 8))) nil))
```