DOC: Add content for Development/Error handling page.

This page was previously empty.  This commits populates it with some
very basic information and a code example of how to use
proj_context_errno and proj_context_errno_string.

Something: because something is better than nothing.

It might be good to include some more fine detail about how PROJ manages
the error state internally, but I don't yet have the expertise in PROJ
internals to write that content myself.

Also the code sample probably has a segfault bug or leaks memory,
because C.

1 files changed, 58 insertions, 0 deletions

diff --git a/docs/source/development/errorhandling.rst b/docs/source/development/errorhandling.rst
index 58199e3f..021d2fb7 100644
--- a/docs/source/development/errorhandling.rst
+++ b/docs/source/development/errorhandling.rst
@@ -4,4 +4,62 @@
 Error handling
 ================================================================================
 
+PROJ maintains an internal error state, which is local to a
+:c:type:`PJ_CONTEXT` thread context.
 
+See :doc:`quickstart` for more information about how to create and use a thread
+context object.
+
+If you receive an abnormal return from a PROJ API function (e.g. a NULL
+pointer) you may wish to discover more information about the error.
+
+In this case you can make a call to :c:func:`proj_context_errno`, passing in
+your thread context.  This will return an integer error code.
+
+If the error code is zero, the last PROJ operation was deemed successful and no
+error has been detected.
+
+If the error code is non-zero, an error has been detected.  You can pass your
+thread context together with this error code to
+:c:func:`proj_context_errno_string` to retrieve a string describing the error
+condition.
+
+A basic example showing how a C program might catch and report errors follows:
+
+.. code-block:: c
+  :caption: errorhandling.c
+  :linenos:
+
+    #include <stdio.h>
+    #include <proj.h>
+
+    int main (void) {
+        PJ_CONTEXT *c;
+        PJ *p;
+        int errno;
+        const char *errstr;
+
+        c = proj_context_create();
+        p = proj_create_crs_to_crs(c, "EPSG:4326", "EPSG:3857", NULL);
+
+        if (p == 0) {
+            /* Something is wrong, let's try to get details ... */
+            errno = proj_context_errno(c);
+            if (errno == 0) {
+                /* This should be impossible. */
+                fprintf(stderr, "Failed to create transformation, reason unknown.\n");
+            } else {
+                errstr = proj_context_errno_string(c, errno);
+                fprintf(stderr, "Failed to create transformation: %s.\n", errstr);
+            }
+            proj_context_destroy(c);
+            return 1;
+        }
+        
+        /* transformation object is valid, do work ... */
+
+        proj_destroy(p);
+        proj_context_destroy(c);
+
+        return 0;
+    }