Small fixes in cmake server logic

Summary:
The patch fixes:

1. Error message about QLocalSocket::ServerNotFoundError when we try to connect to socket which is not already created by cmake server process.
2. Error message about cmake server process crashing when we kill it in the destructor.

Test Plan: Tested on master branch

Reviewers: apol

Reviewed By: apol

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5591