crypto: Improve error checking and reporting

Added checks where necessary to prevent hard crashes and gave
precedence to returning the OpenSSL error strings instead of generic
error strings.

PR-URL: https://github.com/nodejs/node/pull/3753


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>