Adapt existing tests by adding checks for PlantUML 'Syntax Error' messages

[sjoerdtalsma]

Is your feature request related to a problem? Please describe.

Issue #276 remained undetected for long because although we checked the PlantUML sources that were produced by the Doclet, we didn't test whether these sources still produce accurate diagrams.

Describe the solution you'd like

Fortunately checking for illegal syntax is relatively simple: check for the text "Syntax Error" in the resulting .svg diagram files.
Add this condition to all existing tests. This should then automatically trigger a build failure if a feature becomes unsupported in a future PlantUML version.