Clean up `verify_cert` patch.

There are a few major changes here:

1) Added a `OpenSSL._util.exception_from_context_error` to help digest
errors from an native X509StoreCtx object, which is handled quite
differently than the normal handling of errors.

2) Deleted `verify_chain` method, which added application logic atop
native OpenSSL API in favor of `verify_chain`, which exposes native
OpenSSL API.

3) Added tests for `verify_cert`.