Import candidate: `CommandError`

[mih]

Needed by #9

This exception is absolutely essential for DataLad. It may actually deserve its own dedicated place in datasalad

There are features in the datalad-core implementation that seem like an overfit for a basic exception. For example the formatting of JSON-encoded stdout content.

For the purpose of datasalad the type CommandError is much more relevant than the specific user-facing behavior (e.g., to_str()). It would make sense to gut the class to the essentials. Client code can amend it with particular behavior.