scribble: add `--html-tree <n>` mode for rendering to depth <n>

Depth 0 is the same as `--html`, depth 1 is the same as `--htmls`,
and higher depths cause sections and subsections to be rendered
into separate directories when they have their own pages.
scribble HTML rendering merges `style` attributes

This change fixes using `color-property` and `background-color-property`
at the same time, for example.
scribble/manual: generalize `deprecated`
adjust docs on `collection-path`

Use the `deprecated` function, and rephrase in terms of the collection
search path.
distro-build: recognize timeout and exceptions as failure