Add input validation to all scripts

- Should validation be an option or should it be enabled always?
- Do all scripts need a `assert-valid` option, or is it always true for some scripts?

See original request by Tuomas Kassila [on the mailing list](https://groups.google.com/forum/#!topic/daisy-pipeline-dev/Itg4KEAfJAA):

> Hi,
> script dtbook-to-daisy3 is missing --assert-valid option! Not all dtbooks or another input files with diff. scripts are validated files! Could you add --assert-valid true option into next scripts:
>
> dtbook-to-daisy3 script
> html-to-epub3
> dtbook-to-zedai
> epub3-to-daisy202
> zedai-to-html
> daisy3-to-daisy202
> html-to-pef
> zedai-to-epub3
> epub3-to-pef
> daisy202-to-epub3
>
> And the same options into all clients.
>
> Thanks,
>
> Tuomas

Start by reviewing the listed scripts—such as dtbook-to-daisy3, html-to-epub3, and epub3-to-pef—and the corresponding clients to determine how validation options currently work. Resolve whether assert-valid is universal or configurable, then verify that the agreed behavior is consistent across all named scripts and clients.

Written by the indexing model from the issue text.