vdoc: improve error message for non-existing symbols (#9359)

cmd/tools/vdoc/vdoc.v

@@ -330,7 +330,8 @@ fn (mut vd VDoc) generate_docs_from_file() {
 		}
 		outputs := vd.render(out)
 		if outputs.len == 0 {
-			println('No documentation for $dirs')
+			eprintln('vdoc: No documentation found for ${dirs[0]}')
+			exit(1)
 		} else {
 			first := outputs.keys()[0]
 			println(outputs[first])

vlib/v/doc/doc.v

@@ -231,10 +231,6 @@ pub fn (mut d Doc) stmt(stmt ast.Stmt, filename string) ?DocNode {
 			return error('invalid stmt type to document')
 		}
 	}
-	included := node.name in d.filter_symbol_names || node.parent_name in d.filter_symbol_names
-	if d.filter_symbol_names.len != 0 && !included {
-		return error('not included in the list of symbol names')
-	}
 	return node
 }
 
@@ -428,6 +424,13 @@ pub fn (mut d Doc) file_asts(file_asts []ast.File) ? {
 			}
 		}
 	}
+	if d.filter_symbol_names.len != 0 && d.contents.len != 0 {
+		for filter_name in d.filter_symbol_names {
+			if filter_name !in d.contents {
+				return error('vdoc: `$filter_name` symbol in module `$d.orig_mod_name` not found')
+			}
+		}
+	}
 	d.time_generated = time.now()
 }