Prevent exceptions being masked by undefined method errors in ruby 2.4

[zspencer]

@glejeune - This may be a ruby 2.4 idiosynracy, I didn't take the time to diagnose it further.

Essentially, sometimes the status returned from output_and_errors_from_command is nil, which does not respond to zero?; causing the actual commands errors to be masked.

This patches that so the underlying graphviz usage errors can bubble up.

I believe this resolves both https://github.com/glejeune/Ruby-Graphviz/issues/133 and https://github.com/glejeune/Ruby-Graphviz/issues/125

[aguynamedryan]

Ran into the same issue and this PR fixes it for me. Thanks @zspencer. @glejeune, would you be willing to merge this?

[ymarkus]

Can confirm. This makes it usable in my project. Please merge it!

[soberstadt]

This came up for me today too. It exposed an actual error from graphviz (not a helpful error, but at least an error). Please merge this :)