Add a function glp_have_error() to check whether we are currently
handling an error message.

diff -ru glpk-4.55/doc/glpk06.tex glpk-4.55-patched//doc/glpk06.tex
--- glpk-4.55/doc/glpk06.tex	2014-08-22 08:00:00.000000000 +0200
+++ glpk-4.55-patched//doc/glpk06.tex	2015-11-05 10:01:32.244898918 +0100
@@ -303,6 +303,23 @@
 a global jump using the standard function \verb|longjmp|, in which case
 the application program {\it must} call the routine \verb|glp_free_env|.
 
+\subsection{glp\_have\_error --- are we currently handling an error?}
+
+\synopsis
+
+\begin{verbatim}
+   int glp_have_error(void);
+\end{verbatim}
+
+\description
+
+The routine \verb|glp_have_error| returns 0 in normal operation and 1
+if GLPK is currently handling an error.
+
+This can be used by a custom output handler (installed with
+\verb|glp_term_hook|) to determine whether or not the message to be
+displayed is an error message.
+
 \subsection{glp\_alloc --- allocate memory block}
 
 \synopsis
diff -ru glpk-4.55/src/env/env.h glpk-4.55-patched//src/env/env.h
--- glpk-4.55/src/env/env.h	2014-08-22 08:00:00.000000000 +0200
+++ glpk-4.55-patched//src/env/env.h	2015-11-05 09:57:19.049622663 +0100
@@ -161,6 +161,9 @@
 void glp_error_hook(void (*func)(void *info), void *info);
 /* install hook to intercept abnormal termination */
 
+int glp_have_error(void);
+/* are we currently handling an error? */
+
 #define put_err_msg _glp_put_err_msg
 void put_err_msg(const char *msg);
 /* provide error message string */
diff -ru glpk-4.55/src/env/error.c glpk-4.55-patched//src/env/error.c
--- glpk-4.55/src/env/error.c	2014-08-22 08:00:00.000000000 +0200
+++ glpk-4.55-patched//src/env/error.c	2015-11-05 09:59:57.588718768 +0100
@@ -47,6 +47,7 @@
       va_end(arg);
       xprintf("Error detected in file %s at line %d\n",
          env->err_file, env->err_line);
+      env->err_file = NULL;
       if (env->err_hook != NULL)
          env->err_hook(env->err_info);
       abort();
@@ -120,6 +121,28 @@
 }
 
 /***********************************************************************
+*  NAME
+*
+*  glp_have_error - are we currently handling an error?
+*
+*  SYNOPSIS
+*
+*  int glp_have_error(void);
+*
+*  DESCRIPTION
+*
+*  The routine glp_have_error returns 0 in normal operation and 1 if
+*  GLPK is currently handling an error.
+*
+*  This can be used by a term hook to determine whether or not the
+*  message to be displayed is an error message. */
+
+int glp_have_error(void)
+{     ENV *env = get_env_ptr();
+      return env->err_file != NULL;
+}
+
+/***********************************************************************
 *  NAME
 *
 *  put_err_msg - provide error message string
diff -ru glpk-4.55/src/glpk.h glpk-4.55-patched//src/glpk.h
--- glpk-4.55/src/glpk.h	2014-08-22 08:00:00.000000000 +0200
+++ glpk-4.55-patched//src/glpk.h	2015-11-05 09:57:11.291421102 +0100
@@ -839,6 +839,9 @@
 void glp_error_hook(void (*func)(void *info), void *info);
 /* install hook to intercept abnormal termination */
 
+int glp_have_error(void);
+/* are we currently handling an error? */
+
 #define glp_malloc(size) glp_alloc(1, size)
 /* allocate memory block (obsolete) */