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`.
* on some distributions sh is *not* bash, so don't enforce sh
* it's not neccessarey to call the scripts with a shell because the
permissions allow for direct execution
* nitpicking: every part uses $HOME, so change ~ to $HOME for some
calls
