Use `Error::Preprocess` on pp parsing error, instead of `Error::Parse`.

[DaveMcEwan]

• Enable consumers to distinguish between parse errors in the preprocessing and source description stages.
• Error::Preprocess was already defined, so this makes use of it.
• Change the definition of Error::Preprocess to include file and line number, same as before.
• Compatible with current version of svlint, i.e. prints a message like: Error: Preprocess error: Some(("/path/to/file.sv", 55))
• The reported line number is wrong (bug for a later PR), but it's better than nothing.