Set a deprecation deadline for error_state variable?

[Rik]

jwe,

The error_state variable was deprecated in Octave 4.2 (Nov. 2016).  From
the NEWS.4 file

 ** The global error_state variable in Octave's C++ API has been
    deprecated and will be removed in a future version.  Now the error
    and print_usage functions throw an exception
    (octave::execution_exception) after displaying the error message.
    This makes the error and print_usage functions in C++ work more like
    the corresponding functions in the scripting language.

However, the code in error.cc only uses a comment to mark it.

// Deprecated variables and functions.

// This variable is obsolete and always has the value 0.
int error_state = 0;

Shouldn't we mark this with an OCTAVE_DEPRECATED macro or some such so we
remember, eventually, to remove it?  Seems like we could remove it in
version 7 which would be five years after its deprecation.

--Rik