--profile-svg doesn't output an error if graphviz is not installed

[twmr]

When graphviz is not installed and pytest is called with --profile-svg, the following line is output at the end of the pytest logs:

SVG profile in /home/..../myproj/prof/combined.svg.

But this file doesn't exist. After installing graphviz the svg exists. Please output an error when the dot command was not found.

[jackstephenson96]

+1, installed graphviz and still no SVG is generated despite seeing: SVG profile in /home/..../project/prof/combined.svg.

[mikegerber]

Here, combinved.svg was created after installing graphviz.