Install scripts: Improve error handling, cleanup format

This MR is a fix for YT-66.

In particular, following is done:

• fng-install.sh does not report "Update successful" anymore, when some postinstallation scripts fail. This is achieved by saving results of completion (exit codes) of postinstallation script to file. And at the end of execution of fng-install.sh, it inspects the file and reports overall status. Herewith if some script fails, the execution of fng-install.sh continues;
• also, several minor problems reported by shellcheck were fixed;
• fixed a typo in argument name for prepare-logo.sh