utils: Only consider dot data processing fail if dot returns non-zero
dot can output some warnings but still generate the svg output. For example, when Graphviz is not built with Pango or the system has no font installed, dot can complain
Warning: no hard-coded metrics for 'sans-serif'
but still generate the svg. The return code of dot allows us to distinguish errors and warnings, so we can stop treating warnings as hard errors.
I've checked the visualized class hierarchy graphs of glib-2.80.0 with this change applied to gi-docgen, and a Graphviz built w/o Pango. The graphs still look fine.
Fixes #188.