Log warning message when failing to generate graph

README.md

@@ -17,6 +17,7 @@ as well as top-level type aliases, functions, and variables.
 ## Requirements
 
 - Swift 5.2
+- [GraphViz][graphviz] _(optional)_
 
 ## Command-Line Utility
 

Sources/swift-doc/Supporting Types/Components/Relationships.swift

@@ -49,7 +49,7 @@ struct Relationships: Component {
         do {
             return try HypertextLiteral.HTML(String(data: graph.render(using: algorithm, to: .svg), encoding: .utf8) ?? "")
         } catch {
-            logger.error("\(error)")
+            logger.warning("Failed to generate relationship graph for \(symbol.id). Please ensure that GraphViz binaries are accessible from your PATH. (\(error))")
             return nil
         }
     }