use strerror() to generate error message

as suggested by @terrelln .

also:
- hopefully fixed Windows version
- changed the test, so that it passes on non-english OS stdlib errors.

tests/playTests.sh

@@ -187,7 +187,7 @@ $ECHO "test: overwrite input file (must fail)"
 $ZSTD tmp -fo tmp && die "zstd overwrote the input file"
 $ZSTD tmp.zst -dfo tmp.zst && die "zstd overwrote the input file"
 $ECHO "test: properly detect input file does not exist"
-$ZSTD nothere 2>&1 | grep "o such file"
+$ZSTD nothere && die "zstd hasn't detected that input file does not exist"
 
 $ECHO "test : file removal"
 $ZSTD -f --rm tmp