Finished find messagebox is not displayed after finishing the search via "Find Next" (F3) in Regedit

Steps to reproduce:

1. Open Regedit.
2. Click "Edit" -> "Find Next".
3. Type any keyword in the search field that definitely doesn't exist in the registry (e. g., I typed "lol").
4. Click "Find Next".

It will not find any keys/values containing that keyword, and after nothing will found,  Regedit will not notify the user about the search has been finished: regedit_before_2.mp4.
In Windows Regedit, when no (more) items have been found after clicking "Find Next" or just pressing F3 many times, the finished find messagebox is displayed, so then user exactly knows that the search is finished and no more items have been found: MS_regedit_2.mp4.
The problem is, that messagebox is not called in the appropriate part of the code. We just call FindNext function instead.
To fix this, we should to check success of that function, and in case it has been failed, call the messagebox with appropriate strings. This means exactly what we need: when no more items have been found, will appear a message that the search has been finished.

I made a fix for this, and will submit a PR. After my changes, it has the same behaviour as MS Regedit: regedit_after_2.mp4.