Consistently return HassleError and remove unneeded check macros

Our public API becomes a lot cleaner when returning `HassleError`
everywhere, helped by the fact that `HRESULT`s from the low-level COM
API is a newtype with support for functions to convert to `Result<>`
easily.  These can be short-circuit-returned in a more Rust'y way
through the questionmark operator instead of wrapping everything in
large macro calls.  This change is based on a similar approach in the
Ash crate: https://github.com/MaikKlein/ash/pull/339/files

examples/intellisense-tu.rs

@@ -43,17 +43,11 @@ fn main() {
 
     let child_cursors = cursor.get_all_children().unwrap();
 
+    assert_eq!(child_cursors[0].get_display_name().unwrap(), "g_input");
+    assert_eq!(child_cursors[1].get_display_name().unwrap(), "g_output");
     assert_eq!(
-        child_cursors[0].get_display_name(),
-        Ok("g_input".to_owned())
-    );
-    assert_eq!(
-        child_cursors[1].get_display_name(),
-        Ok("g_output".to_owned())
-    );
-    assert_eq!(
-        child_cursors[2].get_display_name(),
-        Ok("copyCs(uint3)".to_owned())
+        child_cursors[2].get_display_name().unwrap(),
+        "copyCs(uint3)"
     );
 
     for child_cursor in child_cursors {