Only return VERIFY_FAILED from a single point

Everything else is a fatal error. Also improve documentation about that for
the vrfy callback.
diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index 5e549f6..31591e2 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -71,7 +71,7 @@
  * Name      ID  Nr of Errors
  * PEM       1   9
  * PKCS#12   1   4 (Started from top)
- * X509      2   19
+ * X509      2   20
  * PKCS5     2   4 (Started from top)
  * DHM       3   9
  * PK        3   14 (Started from top)