Implementations of ENGINEs often don't want to implement every function.
This change adds an error code for those situations.
Change-Id: Id6b7eace36d06ffad7f347f556d942d447d8a2fd
Reviewed-on: https://boringssl-review.googlesource.com/1920
Reviewed-by: Wan-Teh Chang <wtc@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
Initial fork from f2d678e6e89b6508147086610e985d4e8416e867 (1.0.2 beta).
(This change contains substantial changes from the original and
effectively starts a new history.)
