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Some functions (the ones that do I/O) use this crazy three-level error
system (rv, SSL_get_error, and the error queue). Others are more
straightforward and simply use the error queue.

Using SSL_get_error here could result in state from other operations
getting in the way of things.

Since these don't actually do I/O and really can only fail on internal
error, just map to ERR_FAILED rather than try to have so many
MapOpenSSLError variants.

Also treat SSL_export_key_material's return value as a boolean. In
BoringSSL, the calling convention is simpler and documented as such.

BUG=none

Review-Url: https://codereview.chromium.org/2342123002
Cr-Commit-Position: refs/heads/master@{#419006}