Synopsis - Issue Tracker

Currently absence of 'dot' (graphviz) leads to a backtrace.

Ideally Synopsis could:
- search for dot during configure (must-have dot);
- print warning but continue during formatting when dot is not available;
- print error during formatting, requesting a formatter setting to disable dot usage.

rev:1699 enhances the handling of errors, and in
particular, avoids the stacktrace if 'dot' wasn't
found. Instead an error is written.
I don't think a 'configure' step makes sense in
the context of python. However, instead of aborting
the process synopsis should continue without the
graph generation, and print a warning.