Improve error capturing

[logicalparadox]

• Pass actual and expected values upon construction of AssertionError so that testing libraries that check for this can report on it.
• Make assert custom errors more useful.

[andrewcooke]

also, the message. or maybe i am misunderstanding how to use the library? when i have

chai.assert.ok(null, 'some useful error message')

why do i see "Expected null to be truthy" (or similar) instead of the helpful message i added?

[logicalparadox]

What test library are you use?

[andrewcooke]

me? i am actually using chai within code (to catch bugs), not a test framework. the message above was what was shown in the javascript console in chrome.

(it's not a big deal, after lunch i was about to write my own assertion library instead...)

[logicalparadox]

Interesting, certainly not an expected use case (outside of a test framework). Anyway, here is an explanation in case you were wondering: Because of the other interfaces, the AssertionError.message are auto generated. Your custom messages are available at thrownError.summary() if you are try/catching your errors.

Making those custom errors for assert style more useful is within the scope of this ticket.

[logicalparadox]

actual / expected support in a6447e8

will be in 0.5.0 release