code: public static <T> Mono<T> illegalState(String format, Object... args) {
          String message = String.format(format, args);
          return Mono.error(new IllegalStateException(message));
      }
docs: Returns a {@link Mono} containing an {@link IllegalStateException} with the configured message
  
  @param format A <a href="../util/Formatter.html#syntax">format string</a>
  @param args   Arguments referenced by the format specifiers in the format string.  If there are more arguments than format specifiers, the extra arguments are ignored.  The number of arguments
  is variable and may be zero.  The maximum number of arguments is limited by the maximum dimension of a Java array as defined by <cite>The Java&trade; Virtual Machine
  Specification</cite>. The behaviour on a {@code null} argument depends on the <a href="../util/Formatter.html#syntax">conversion</a>.
  @param <T>    the type of the {@link Mono} being converted
  @return a {@link Mono} containing the error