Catch only UnicodeError for string decode failure.

The default exception (which we use) for decode
failures is UnicodeError or a subclass of it.

This fixes a unittest that forces RuntimeError. We
shouldn't use blanket except blocks either way.

bson/__init__.py

@@ -87,7 +87,7 @@ def _make_c_string(string, check_null=False):
         try:
             string.decode("utf-8")
             return string + "\x00"
-        except:
+        except UnicodeError:
             raise InvalidStringData("strings in documents must be valid "
                                     "UTF-8: %r" % string)