Plugin zsh_reload: make sure cache dir exists

The [`zsh_reload`](https://github.com/robbyrussell/oh-my-zsh/tree/master/plugins/zsh_reload) plugin uses the `$ZSH/cache` directory, which does not exist on a fresh OMZ install. Hence the `src` function will fail to recompile the compdump:
```
re-compiling /home/ncanceill/.oh-my-zsh/cache/zcomp-epsilon.zwc: zrecompile:zcompile:133: can't write zwc file: /home/ncanceill/.oh-my-zsh/cache/zcomp-epsilon.zwc
re-compiling /home/ncanceill/.oh-my-zsh/cache/zcomp-epsilon.zwc: failed
```

This PR adds a check for it.

@mcornella @eMxyzptlk any objections?
Ok, it's done! I love it when you double-check my work and help find all my mistakes ;-)