This is not documented, but we have added some HTTP header in the HTTP response.
For example, if you go to http://www.plantuml.com/plantuml/png/SyfFKj2rKt3CoKnELR1Io4ZDoSddAid8B5O0eiGyXTIYelmY0000
You will have the following headers:
X-PlantUML-Diagram-Error: Syntax Error?, Syntax Error?
X-PlantUML-Diagram-Error-Line: 2, 2
This is something you can use for your own need. Is this what you are looking for ?