improve error messages for missing conventions

[PascalSenn]

Changes error message for missing default convention from

No filter convention found for scope `none`

to

No default filter convention found. Call `AddFiltering()` on the schema builder.

and for scope specific error messages from

No filter convention found for scope `Foo`

to

No filter convention found for scope `Foo`. Register a convention with `AddConvention<IFilterConvention, TYourConvention>("Foo")` on the schema builder.

[sonarcloud]

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