vfmt: change all '$expr' to '${expr}' (#16428)

vlib/datatypes/fsm/fsm.v

@@ -32,7 +32,7 @@ pub fn (mut s StateMachine) set_state(name string) ? {
 	if name in s.states {
 		s.current_state = name
 	}
-	return error('unknown state: $name')
+	return error('unknown state: ${name}')
 }
 
 pub fn (mut s StateMachine) get_state() string {

vlib/datatypes/fsm/tools/fsm_graph.v

@@ -4,7 +4,8 @@ import flag
 pub fn read_file(file string) ?[]string {
 	if os.is_file(file) {
 		text := os.read_lines(file) or {
-			return error(@MOD + '.' + @STRUCT + '.' + @FN + ' Could not read "$file": "$err.msg()"')
+			return error(@MOD + '.' + @STRUCT + '.' + @FN +
+				' Could not read "${file}": "${err.msg()}"')
 		}
 		return text
 	}