IMN-784 - Fixing attribute duplicate errors

[ecamellini]

Closes IMN-784

Before this PR

The current attributeDuplicate error is misleading:

✅ in the case of verified/declared attribute creation it is thrown when an attribute with the same name already exists, with the message: ApiError during Attribute creation with name ${attributeName}

❌ in the case of certified attribute creation, the error is thrown only when an attribute with the same name and code already exists, but the error message is the same

Fix in this PR:

• I created a dedicated error for the second case, with a more meaningful message
• I also improved the tests to check that the check on the names is case insensitive
• The code for both errors is still the same, so that the error mappers still work as expected