vpp-api: Add context to VPPStatsIOError.

* Provide context information and return meaningful information.
VPPStatsIOError: Stat segment client connection returned: -2 Stat client socket open but couldn't connect.

Change-Id: I14ea35c58452a2cb7598a7efd136f0c7e1f5743a
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>
diff --git a/src/vpp-api/client/stat_client.c b/src/vpp-api/client/stat_client.c
index bb7f240..2cf53d5 100644
--- a/src/vpp-api/client/stat_client.c
+++ b/src/vpp-api/client/stat_client.c
@@ -124,14 +124,14 @@
       0)
     {
       close (sock);
-      return -1;
+      return -2;
     }
 
   if ((mfd = recv_fd (sock)) < 0)
     {
       close (sock);
       fprintf (stderr, "Receiving file descriptor failed\n");
-      return -1;
+      return -3;
     }
   close (sock);
 
@@ -142,13 +142,13 @@
   if (fstat (mfd, &st) == -1)
     {
       perror ("mmap fstat failed");
-      return -1;
+      return -4;
     }
   if ((memaddr =
        mmap (NULL, st.st_size, PROT_READ, MAP_SHARED, mfd, 0)) == MAP_FAILED)
     {
       perror ("mmap map failed");
-      return -1;
+      return -5;
     }
 
   sm->memory_size = st.st_size;