Add dictionary of pre-defined error messages to SSLContextError.
Add dictionary of pre-defined error messages to SSLContextError.

 * This help us make informative messages for the user and logs, later when
   setUp() is called, so that we can explain, for example, why we can't test
   TLS and SSLv2/SSLv3 simultaneously.

Karthikeyan-kkk/ooni-probe,0xPoly/ooni-probe,lordappsec/ooni-probe,Karthikeyan-kkk/ooni-probe,kdmurray91/ooni-probe,Karthikeyan-kkk/ooni-probe,kdmurray91/ooni-probe,0xPoly/ooni-probe,0xPoly/ooni-probe,kdmurray91/ooni-probe,juga0/ooni-probe,Karthikeyan-kkk/ooni-probe,juga0/ooni-probe,juga0/ooni-probe,lordappsec/ooni-probe,kdmurray91/ooni-probe,lordappsec/ooni-probe,0xPoly/ooni-probe,juga0/ooni-probe,lordappsec/ooni-probe