Error cases need better documentation, they are rejected Promises

[ColinEberhardt]

In a number of instance, API methods are described as returning errors with a given error string.

For example, the description for open is as follows:

If opening errors, it returns an Error with a string from the OpenError enumeration.

The open method returns a Promise, as a result, it doesn't return an error, instead, the Promise is rejected with a resultant value.

Notably, a few container implementations don't reject Error types, instead they reject with the resultant string value.

[ColinEberhardt]

See also: finos/FDC3-conformance-framework#53