Feature: Expose OnCertificateError to allow host application to decide whether or not it should continue

This change adds `OnCertificateError` to the  `IRequestHandler` interface. 

Implementers that are not interested in handling certificate errors should return `false` to indicate that the browser must not proceed with an invalid certificate. 

However, applications that what to proceed (after prompting the user) can now implement `OnCertificateError` and return `true` to proceed even if there is a certificate error.