Add fixes for setup script

[Mudskipper875]

Use $0 instead of pwd command

• pwd will cause errors if the script is launched from some other directory.

False positive with [[ -f "$ROFI_DIR/config.rasi" ]]

• This may give a false successful installation message if the user's original 'config.rasi' is still in the directory.
• Add set -eu which will cause the script to exit if there are any errors to prevent data loss.
• Add trap command to print error message on script exit.

Some more fixes/changes

• Check if XDG user direcotries are set.
• Simplify script by replacing if statements with || and case.