Clean up error handling in declarative shadow DOM code

This CL started as an implementation of exception throwing from
the parsing code for DSD. However, as discussed in the issue:

  https://github.com/whatwg/html/issues/10527

the desire is not to add such exceptions, mostly for fear of
opening new XSS endpoints. So that bit has been ripped out of
the CL, and what's left is a bit of a cleanup that at least
moves all errors (including wrong-mode-string) to a central
place in `ErrorMessageForAttachShadow`. This also allowed me
to remove the `DeclarativeShadowRootMode` enum and some
related code.

I also added a few small explicit tests that exceptions are
not thrown/fired during HTML parsing.

Since the spec says exceptions should be fired in this case,
I will put up a spec PR soon. For now, though, this is not
a behavioral change for Chrome, so no need to do an I2S or
anything.

The spec, currently:
 -https://html.spec.whatwg.org/#parsing-main-inhead

Start at `A start tag whose tag name is "template"`, then Otherwise,
and then point 8, which is:

> Attach a shadow root with declarative shadow host element, mode,
> clonable, serializable, delegatesFocus, and "named". If an exception
> is thrown, then catch it, report it for declarative shadow host
> element's relevant global object, insert an element at the adjusted
> insertion location with template, and return.


Change-Id: I0a8e2c8fa1301efdd28f9bb786f9ca8d3596c2fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5750587
Auto-Submit: Mason Freed <masonf@chromium.org>
Commit-Queue: Di Zhang <dizhangg@chromium.org>
Reviewed-by: Di Zhang <dizhangg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1339961}