Support non-english errors with non-ascii chars.

Libgit2 partially forwards OS error message texts.
On non-english Windows OSes these errors may contain non-ascii characters (i.e. umlauts).
To avoid that a UnicodeDecodeError is raised the error message is interpreted as UTF-8.
The solution should not be necessary on linux/osx as they return always ascii (as far as I know).
Thus this solution will not change the behaviour on linux/osx.

pygit2/errors.py

@@ -40,7 +40,7 @@ def check_error(err, io=False):
     # Error message
     giterr = C.giterr_last()
     if giterr != ffi.NULL:
-        message = ffi.string(giterr.message).decode()
+        message = ffi.string(giterr.message).decode('utf8')
     else:
         message = "err %d (no message provided)" % err