Non informative log message in case of a non-existing user authentication

[orenbm]

Summary

In case a user sends an authn request with the authn authenticator,
and the user is not defined in Conjur, the log message will show:
Authentication Error: #<NoMethodError: undefined method valid_origin?' for nil:NilClass>`.

This is not informative and doesn't tell the user the real story. We should change
the message to:
Authentication Error: #<Errors::Authentication::Security::RoleNotFound: CONJ00007E ‘cucumber:user:non-existing-user’ not found>

Steps to Reproduce

Steps to reproduce the behavior:

1. send an authn request with a user that doesn't exist
2. Read the logs

Expected Results

An informative message is written: Authentication Error: #<Errors::Authentication::Security::RoleNotFound: CONJ00007E ‘cucumber:user:non-existing-user’ not found>

Actual Results (including error logs, if applicable)

message shows: Authentication Error: #<NoMethodError: undefined method `valid_origin?' for nil:NilClass>