minor #15501 [DX] make doctrine mappings compiler pass exception message more understandable (dbu)

This PR was merged into the 2.7 branch.

Discussion
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[DX] make doctrine mappings compiler pass exception message more understandable

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

Commits
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32f92cc make doctrine mappings compiler pass exception message more understandable

src/Symfony/Bridge/Doctrine/DependencyInjection/CompilerPass/RegisterMappingsPass.php

@@ -231,7 +231,7 @@ abstract class RegisterMappingsPass implements CompilerPassInterface
             }
         }
 
-        throw new ParameterNotFoundException('None of the managerParameters resulted in a valid name');
+        throw new ParameterNotFoundException('Could not determine the Doctrine manager. Either Doctrine is not configured or a bundle is misconfigured.');
     }
 
     /**