Add specific SSL error code for unexpected CIDs

Currently, the stack silently ignores DTLS frames with an unexpected CID.
However, in a system which performs CID-based demultiplexing before passing
datagrams to the Mbed TLS stack, unexpected CIDs are a sign of something not
working properly, and users might want to know about it.

This commit introduces an SSL error code MBEDTLS_ERR_SSL_UNEXPECTED_CID
which the stack can return in response to an unexpected CID. It will
conditionally be put to use in subsequent commits.
diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index bee0fe4..765fd42 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -100,8 +100,8 @@
  * ECP       4   10 (Started from top)
  * MD        5   5
  * HKDF      5   1 (Started from top)
- * CIPHER    6   8
- * SSL       6   23 (Started from top)
+ * CIPHER    6   8 (Started from 0x6080)
+ * SSL       6   24 (Started from top, plus 0x6000)
  * SSL       7   32
  *
  * Module dependent error code (5 bits 0x.00.-0x.F8.)