Elevate warning to errors for unknown attributes

[filimarc]

Describe the work done

Replace ConfigureWarning with ConfigureError for unknown attributes

List which issues this resolves:

closes #787

Tasks

• Added tests
• Updated documentation

[Helveg]

You'll also have to update the tests to reflect the change. Some test cases that should be covered:

• Setting an attribute
• Raise an error when setting an unknown attribute is set
• Raise an error when trying to set a readonly attribute

[Helveg]

• Check the removed test
• Add a test that triggers the readonly property error

[Helveg]

Great! Use the appropriate assertIn idiom for the new test and we're done :)