prettify error message for catch-all conflict with existing path segm…

…ent (gin-gonic#2934)

tree.go

@@ -349,7 +349,12 @@ func (n *node) insertChild(path string, fullPath string, handlers HandlersChain)
 		}
 
 		if len(n.path) > 0 && n.path[len(n.path)-1] == '/' {
-			panic("catch-all conflicts with existing handle for the path segment root in path '" + fullPath + "'")
+			pathSeg := strings.SplitN(n.children[0].path, "/", 2)[0]
+			panic("catch-all wildcard '" + path +
+				"' in new path '" + fullPath +
+				"' conflicts with existing path segment '" + pathSeg +
+				"' in existing prefix '" + n.path + pathSeg +
+				"'")
 		}
 
 		// currently fixed width 1 for '/'