Draft web platform structure

src/platform_impl/web/error.rs

@@ -0,0 +1,10 @@
+use std::fmt;
+
+#[derive(Debug)]
+pub struct OsError(pub String);
+
+impl fmt::Display for OsError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}