Silent failure when output directory does not exist

[david-err]

Hi, great library.
A small issue I ran into, is that tplant will not throw an error/warning when the output directory does not exist. It simply says Done in x secs. while nothing is generated.

In my case, I wanted to output to reports/uml/uml.svg. reports/uml did not exist, reports/xyz did, though.

Suggested solution:

1. Generate output directory if not exists
2. Warn/error when output dir could not be created.

Cheers