Add proj_errno_string function

Equivalent to pj_strerrno.

docs/source/development/reference/functions.rst

@@ -266,7 +266,15 @@ Change the error-state of :c:data:`P` to `err`.
     :param PJ* P: Transformation object.
     :param int err: Error code.
 
+.. c:function:: const char* proj_errno_string(int err)
 
+    Get a text representation of an error number.
+
+    :param int err: Error number.
+
+    :returns: :c:type:`const char*` String with description of error.
+
+    .. note:: Available from version 5.1.0.
 
 Info functions
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

src/pj_strerrno.c

@@ -1,10 +1,11 @@
 /* list of projection system pj_errno values */
-#include "projects.h"
 #include <stdio.h>
 #include <errno.h>
 #include <string.h>
+#include "proj.h"
+#include "projects.h"
 
-    static const char * const
+static const char * const
 pj_err_list[] = {
     "no arguments in initialization list",                             /*  -1 */
     "no options found in 'init' file",                                 /*  -2 */
@@ -96,3 +97,7 @@ char *pj_strerrno(int err) {
     sprintf( note, "invalid projection system error (%d)", (err > -9999)? err: -9999);
     return note;
 }
+
+const char* proj_errno_string(int err) {
+    return pj_strerrno(err);
+}

src/proj.h

@@ -349,6 +349,7 @@ int  proj_errno (const PJ *P);
 int  proj_errno_set (const PJ *P, int err);
 int  proj_errno_reset (const PJ *P);
 int  proj_errno_restore (const PJ *P, int err);
+const char* proj_errno_string (int err);
 
 /* Scaling and angular distortion factors */
 PJ_FACTORS proj_factors(PJ *P, PJ_COORD lp);