zshrc: avoid broken zsh startup if directories within .zdirs / DIRSTACKFILE can't be read

[mika]

We ran into this problem on a customer system, where a NFS mountpoint was referenced in the .zdirs file (via DIRSTACKFILE feature enabled by default), while the NFS share wasn't available when trying to login via SSH. Only once the zdirs file got updated/removed, we could login via ssh to our ZSH again.

We need to ensure that the default behavior of grml-zshrc can't cause such a behavior, reporting to not forget about it. :)