Fixed bogus warning on public key not being found

When using tg-cli from a packaged version that has installed secret.pub under /etc, tg-cli complains that it can't find tg-server.pub on its own directory (this is met only when executing from the source tree), although the public key is then found under /etc/telegram/secret.pub and the program executes gracefully afterwards. That warning is confusing so I've changed the code so that it first checks for the public key under /etc. On failure, it'll warn that it hasn't found the key under /etc, but it then looks for the file on its directory and issues a message if it has found it. If that's not the case, then, the program is interrupted.