This will be useful for the SSL stack to properly resurface handshake
failures. Leave this in a private header and, along the way, hide the
various types.

(ERR_NUM_ERRORS didn't change in meaning. The old documentation was
wrong.)

Bug: 206
Change-Id: I4c6ca98d162d11ad5e17e4baf439a18fbe371018
Reviewed-on: https://boringssl-review.googlesource.com/21284
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Steven Valdez <svaldez@google.com>