minor #15920 Guard minor tweaks (weaverryan)

This PR was merged into the 2.8 branch.

Discussion
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Guard minor tweaks

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Various completely minor things, most from suggestions on #14673

Commits
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869d5a7 tweaking message related to configuration edge case that we want to be helpful with
da4758a Minor tweaks - lowering the required security-http requirement and nulling out a test field

Author: fabpot

DependencyInjection/Security/Factory/GuardAuthenticationFactory.php

@@ -94,7 +94,7 @@ private function determineEntryPoint($defaultEntryPointId, array $config)
             // explode if they've configured the entry_point, but there is already one
             if ($config['entry_point']) {
                 throw new \LogicException(sprintf(
-                    'The guard authentication provider cannot use the "%s" entry_point because another entry point is already configured by another provider! Either remove the other provider or move the entry_point configuration as a root key under your firewall',
+                    'The guard authentication provider cannot use the "%s" entry_point because another entry point is already configured by another provider! Either remove the other provider or move the entry_point configuration as a root key under your firewall (i.e. at the same level as "guard").',
                     $config['entry_point']
                 ));
             }