LibJS doesn't store stacks for exception objects, so this
only amends test-common.js's __expect() with an optional
`details` function that can produce a more detailed error
message, and it lets test-js.cpp read and print that
error message. I added the optional details parameter to
a few matchers, most notably toBe() where it now prints
expected and actual value.
It'd be nice to have line numbers of failures, but that
seems hard to do with the current design, and this is already
much better than the current state.