Make it clear it's an attribute name by quoting.

Trivial change, quoting the attribute name in the `Attribute '$name' must be provided before calling reader` error message, to make it clear it's the name of an attribute.

Reasoning for this: we have a class with an attribute named `content`.  So, the error we got was "Attribute content must be provided before calling reader" - two different devs both misunderstood it, wondering which attribute was missing its content and going off to figure it out, rather than immediately realising "oh, it's the `content` attribute".

I think this trivial change would have avoided that confusion for us, and will help avoid confusion for others.