src/main/java/org/fest/assertions/Fail.java
protected static void failWithMessage(@Nullable String customErrorMessage, Throwable realCause) {
    if (customErrorMessage != null) {
      fail(customErrorMessage, realCause);
    }
  }
protected static void failWithMessage(@Nullable String customErrorMessage, Throwable realCause) {
    if (customErrorMessage != null) {
      fail(customErrorMessage, realCause);
    }
  }
Throws an {@code AssertionError} only if the given custom message is not {@code null}.

@param customErrorMessage the custom error message.
@param realCause          cause of the error.
@throws AssertionError only if the custom error message is not {@code null}.
@since 1.2