Fix bugs in two examples + Fix `master` CI (#820)

* Enable debug output for `contract-transfer` example

It uses `debug_println!`.

* Enable sending payments to `fn invoke_transaction()`

It needs to be possible to send value with the transaction,
because `t.transferred_value` in this function could be set
to > 0.

* Rename `Error::UnknownError` to `Error::Unknown`

Clippy fails otherwise:

	= note: `-D clippy::enum-variant-names` implied by `-D warnings`
	= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#enum_variant_names

* Revert "Enable sending payments to `fn invoke_transaction()`"

This reverts commit 9b051d2c.

* Require sending transaction payment to `fn invoke_transaction()`

It needs to be possible to send value with the transaction,
because `t.transferred_value` in this function could be set
to > 0.

* Fix interpunctuation

* Introduce `ink-debug` feature

* Revert "Introduce `ink-debug` feature"

This reverts commit e61673a5.
21 jobs for master in 30 minutes and 33 seconds (queued for 3 seconds)