Improve error message for starting docker cmdline code executor when docker isnt running

[jackgerrits]

The error message given in this case should point the user more specifically to the potential problem

[MohMaz]

@jackgerrits please feel free to assign this to me, I'll try to take a look

[gziz]

Closing this, as we have merged https://github.com/microsoft/autogen/pull/4600