Fix faulty shared test example

In the `has errors` shared test example, we were passing in a string to
check against the errors but not actually checking it.

This meant you could add any string and the test would always pass,
provided an error existed.

Therefore updating the shared example to actually accept a parameter,
check the error is returned and update the messages to reflect what the
class is actually returning to users.

spec/lib/link_checker_spec.rb

@@ -10,9 +10,10 @@
       end
     end
 
-    shared_examples "has errors" do
+    shared_examples "has errors" do |error = nil|
       it "should have errors" do
         expect(subject.errors).to_not be_empty
+        expect(subject.errors).to include(error) if error
       end
     end
 
@@ -202,42 +203,42 @@
     context "401 status code" do
       let(:uri) { "http://www.not-gov.uk/401" }
       before { stub_request(:get, uri).to_return(status: 401) }
-      include_examples "has errors", "401 error (page requires login)"
+      include_examples "has errors", "A login is required to view this page."
       include_examples "has no warnings"
     end
 
-    context "403 status code" do
+    context "403 status code without Cloudflare challenge header" do
       let(:uri) { "http://www.not-gov.uk/403" }
       before { stub_request(:get, uri).to_return(status: 403) }
-      include_examples "has errors", "403 error (page requires login)"
+      include_examples "has errors", "A login is required to view this page."
       include_examples "has no warnings"
     end
 
     context "404 status code" do
       let(:uri) { "http://www.not-gov.uk/404" }
       before { stub_request(:get, uri).to_return(status: 404) }
-      include_examples "has errors", "404 error (page not found)"
+      include_examples "has errors", "This page was not found (404)."
       include_examples "has no warnings"
     end
 
     context "410 status code" do
       let(:uri) { "http://www.not-gov.uk/410" }
       before { stub_request(:get, uri).to_return(status: 410) }
-      include_examples "has errors", "410 error (page not found)"
+      include_examples "has errors", "This page was not found (404)."
       include_examples "has no warnings"
     end
 
     context "an unspecified 4xx status code" do
       let(:uri) { "http://www.not-gov.uk/418" }
       before { stub_request(:get, uri).to_return(status: 418) }
-      include_examples "has errors", "418 error (page is unavailable)"
+      include_examples "has errors", "This page is unavailable (418)."
       include_examples "has no warnings"
     end
 
     context "5xx status code" do
       let(:uri) { "http://www.not-gov.uk/500" }
       before { stub_request(:get, uri).to_return(status: 500) }
-      include_examples "has errors", "500 (server error)"
+      include_examples "has errors", "This page is responding with an error (500) and won't work for users."
       include_examples "has no warnings"
     end