Sheets fixes

[brafdlog]

• Fix hash column (the current code is fragile because it assumes the which column is the hash column. Will think of a way to make this safer)
• Throw clear error if no sheet with the expected name exists

[baruchiro]

OK, no big change, but note that with the Frontend I already implemented, I have an option to create a new Spreadsheet, so of course if you're creating a new one, you will not have even the Spreadsheet.

This lead me to the open issue- #120

I don't care if you merging this, but I think it's not relevent. Unless you think otherwise so that we need to talk.

Written from Github CLI so no auto English fixes

[brafdlog]

OK, no big change, but note that with the Frontend I already implemented, I have an option to create a new Spreadsheet, so of course if you're creating a new one, you will not have even the Spreadsheet.

Thats true but in any case, the spreadsheet is something the user can change and if deletes/renames the sheet we will fail and I would like to get a clear error so I think it is still relevant.