add: show both differences at unknown header error

src/main.py

@@ -48,14 +48,19 @@ def create_items(file: UploadFile = File(...), db: Session = Depends(get_db)):
 
             # we have to construct a collection of known headers anyway,
             # might as well make it a set
-            unknown_headers = set(sheet["header"]) - set(
+            sheet_header_set = set(sheet["header"])
+            unknown_headers = sheet_header_set - set(
                 dbh.spreadsheet for dbh in crud.get_headers(db=db)
             )
+            missing_headers = (
+                set(dbh.spreadsheet for dbh in crud.get_headers(db=db))
+                - sheet_header_set
+            )
             if unknown_headers:
                 raise HTTPException(
                     status_code=422,
-                    detail="Unknown headers in the spreadsheet: {}. Check the coordinated format".format(
-                        unknown_headers
+                    detail="Unknown headers in the spreadsheet: {}. Missing headers from the database: {}. Check the coordinated format".format(
+                        unknown_headers, missing_headers
                     ),
                 )