The private key callback may not push one of its own (it's possible to
register a custom error library and whatnot, but this is tedious). If
the callback does not push any, we report SSL_ERROR_SYSCALL. This is not
completely wrong, as "syscall" really means "I don't know, something you
gave me, probably the BIO, failed so I assume you know what happened",
but most callers just check errno. And indeed cert_cb pushes its own
error, so this probably should as well.

Update-Note: Custom private key callbacks which push an error code on
    failure will report both that error followed by
    SSL_R_PRIVATE_KEY_OPERATION_FAILED. Callbacks which did not push any
    error will switch from SSL_ERROR_SYSCALL to SSL_ERROR_SSL with
    SSL_R_PRIVATE_KEY_OPERATION_FAILED.

Change-Id: I7e90cd327fe0cbcff395470381a3591364a82c74
Reviewed-on: https://boringssl-review.googlesource.com/25544
Reviewed-by: Adam Langley <agl@google.com>