Update admin.py on Windows (see #10095)

- Remove (almost) all instances of sc.exe
- Update messages displayed to user when managing Windows service
- Catch return value from ServiceStart (remove string comparisons)
Unify the try/finally blocks around win32 calls
Fix non-existent user error case
Merge pull request #589 from bpindelski/10095_admin_sc

Improvements to windows service user (see #10095)