Add clearer error message for when Redundancy date is too far in the past

The original message was not very clear and caused confusion for both
users entering out of range dates, and developers investigating unrelated
issues. This new error text is designed to clarify the problem.

Author: Tetrino

app/flows/calculate_your_redundancy_pay_flow/questions/date_of_redundancy.erb

@@ -5,3 +5,7 @@
 <% text_for :hint do %>
   Use the original redundancy date even if your notice is brought forward, you’re paid in lieu of notice or made redundant after trialing a new job.
 <% end %>
+
+<% text_for :error_message do %>
+  The date you were made redundant must be on or after <%= SmartAnswer::Calculators::RedundancyCalculator.first_selectable_date.strftime("%-d %B %Y") %>.
+<% end %>