SSL reason codes corresponding to alerts have special values. Teach
make_errors.go that values above 1000 are reserved (otherwise it will assign
new values in that namespace). Also fix all the existing reason codes which
corresponded to alerts.
Change-Id: Ieabdf8fd59f4802938616934e1d84e659227cf84
Reviewed-on: https://boringssl-review.googlesource.com/1212
Reviewed-by: Adam Langley <agl@google.com>
