ExcelWorkWeekSupplier: Improve stability

Right now the columns (for Home Office time, On-site time and so on) and rows (Monday, ..., Sunday) are hardcoded via constants. Even though some aspects will have to be hardcoded in some way, there are better ways (probably via Column-/Row-Names) to do this. At least the process should be more error resistant and log the error, if the given file can't be processed