fix: allow spaces in current working directory on certificate generation

[Nicceboy]

If the current working directory includes spaces (even if it should not...), the self-signed cert generation script will fail.

Added quotes for the variable use and also set -e to fail script fast on error.

[gecube]

lgtm

@Nicceboy also please consider to switching of generating certificates with cert-manager #577

[gecube]

Also I prefer "${DIR}" syntax over $DIR or even "$DIR"... and for all possible environment variables.
Probably we should pass the shell scripts through some kind of shell linter. No idea which one.