Clean up errors thrown in data.py and related tests

[heubi95]

• Throw ValueError, when delimiters are equal
• Throw positive written exceptions
• Adapt related tests at test_data.py

[roskakori]

This is mostly complete, however we still need a test case for each raise errors.InterfaceError.

[roskakori]

Done. According to https://coveralls.io/files/419671553, the current tests cover all raise statements in data.py.