Print to `stderr` when a graphviz file can't be written
Print to `stderr` when a graphviz file can't be written

`warn` prints nothing by default

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust