Sends the `error_trace` parameter with all requests sent by the
yaml test framework, including the doc snippet tests. This can be
overridden by settings `error_trace: false`. While this drift's
core's handling of the yaml tests from the client's slightly this
should only be a problem for tests that rely on the default value,
both of which I've fixed by setting the value explicitly.
This also escapes `\n` and `\t` in the `Stash dump on failure` so
the `stack_trace` is more readable.
Also fixes `RestUpdateSettingsAction` to not think of the `error_trace`
parameter as a setting.
You can use `Debug.explain(someObject)` in painless to throw an
`Error` that can't be caught by painless code and contains an
object's class. This is useful because painless's sandbox doesn't
allow you to call `someObject.getClass()`.
Closes#20263
Nik Everett