Return a Result<Option<Message>> for get_draft

The function can fail, so we need to still have an error return as
well as a no-draft return.

Add tests.

deltachat-ffi/src/lib.rs

@@ -553,12 +553,23 @@ pub unsafe extern "C" fn dc_get_draft(context: *mut dc_context_t, chat_id: u32)
     }
     let context = &*context;
 
-    if let Some(message) = chat::get_draft(context, chat_id) {
-        let ffi_msg = MessageWrapper { context, message };
-        return Box::into_raw(Box::new(ffi_msg));
-    };
-
-    ptr::null_mut()
+    match chat::get_draft(context, chat_id) {
+        Ok(Some(draft)) => {
+            let ffi_msg = MessageWrapper {
+                context,
+                message: draft,
+            };
+            Box::into_raw(Box::new(ffi_msg))
+        }
+        Ok(None) => ptr::null_mut(),
+        Err(err) => {
+            error!(
+                context,
+                "Failed to get draft for chat #{}: {}", chat_id, err
+            );
+            ptr::null_mut()
+        }
+    }
 }
 
 #[no_mangle]