create tests to check all Raised errors

[shimwell]

ValueErrors are raised when:

• the material_tag is over 28
• the material_tag is missing from the files_with_tags arg
• file name does not end with .stp .step or .sat
• filename is missing from the files_with_tags arg
• when cubit can't be imported from the specified path

We could do with tests for all of these situations to see if the error really triggers the correct raised error.

Example on how to do this can be found here https://github.com/fusion-energy/paramak/blob/8b8b6e94b41899a2aefaa7fc616b2fb47179485a/tests/test_shape.py#L50-L56

We should probably also raise a warning when no graveyard is present