Provide withContext, because, or similar for expectObservable

[AndyAtTib]

It would be exceptionally useful for building larger test suites to be able to provide context for a test failure.

With jasmine, you can do

expect(true).withContext('expectation 1').toBe(true);
expect(false).withContext('expectation 2').toBe(true);

re: "why" here are prior discussions for similar features:

• Adding because descriptions to expect failures jasmine/jasmine#641
• toEqual doesn't display given failure message jasmine/jasmine#1457