* Fix output of wrongly outputted error

The "Tombstoned a contract that is below the subsistence threshold: {:?}" was
triggered when too few balance was provided. It was a false alarm.

* Fix return of wrong code_len

* Split up `NotCallable` into more fine grained errors

* Fix typos in docs

Co-authored-by: Keith Yeung <[email protected]>

* RentNotPayed -> RentNotPaid

* Fix typo: payed -> paid

It is OK to change the in-storage field name because:

1. The SCALE encoding is not based on names only on position.
2. The struct is not public (only to the crate).

Co-authored-by: Keith Yeung <[email protected]>