* constellation: crash to a new “sad tab” page
* check in resources/crash.html
* use a separate enum variant instead of keying on reason
* fmt + tidy
* rename Resource::Crash to Resource::CrashHTML
* clean up crash page and add details (reason + backtrace)
* avoid repeating crash errors in script::script_thread warn log
* make new LoadData init more idiomatic
* clarify comments and new fields
* fix doc comment style
Previously, the `tests` feature flag of the `embedder_traits` crate
caused it and every crate recursively depending on it to be built twice.
This feature flag was used to provide a specific set of "resources"
when running tests. Instead, this commits overrides the `main()` function
of the test harness to change resources at runtime before running any test.
This is done by adding a dependency that has `name = "test"` in its
`[lib]` section of `Cargo.toml`. This overrides the crate found by
`extern crate test;` in code generated by `rustc --test`.
