Remove a number of unwraps. In some cases, a `?` just worked as is. I also made it possible to use `?` to go from Result<OutputStream, ShellError> to OutputStream. Finally, started updating PerItemCommand to be able to use the signature deserialization logic, which substantially reduces unwraps.
This is still in-progress work, but tests pass and it should be clear to merge and keep iterating on master.