Add missing errors codes for alerts.

This gives inappropriate_fallback and close_notify sent during the handshake
error strings. It'd also avoid having to write
  case SSL_AD_REASON_OFFSET + SSL_AD_CLOSE_NOTIFY:
in Chromium.

Change-Id: I42123d5452eb7843ead883d112e58b3f087d3067
Reviewed-on: https://boringssl-review.googlesource.com/1780
Reviewed-by: Adam Langley <agl@google.com>