py/objstr: Handle "errors" param to str() constructor.
py/objstr: Handle "errors" param to str() constructor.

Only "strict" value (default) is explicitly recognized. Any other value
just skips validity check, and uses all input bytes in the resulting string.
Thus, is a kind of cross of "ignore" and "surrogateescape" functionality
(never raised UnicodeError, like "ignore", and offers roundtrip, like
"surrogateescape").

Change-Id: Ie7ebc0beca5d6e26f2667db6fe349f68bceca84f

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython