s: Halt build on pfsc- directives with bad opt block in Sphinx pages

[skieffer]

It will likely be a common error in Sphinx pages, to have an invalid option block in pfsc- directives whenever a PF-JSON object is the value of any of the data fields. This is because there may be a tendency to indent the closing brace in line with the option name, like this:

.. pfsc-chart::
    :view: Thm.C
    :viewOpts: {
        transition: false,
    }

whereas in fact it needs to be indented deeper than the option name, like this:

.. pfsc-chart::
    :view: Thm.C
    :viewOpts: {
        transition: false,
        }

Up to now, an error of this kind, which Sphinx calls an "invalid option block" is simply reported by Sphinx as a warning, which does not halt the build process. In such a case, the widget simply produces no HTML whatsoever. This means you can have a "successful" build, and yet wind up with a Sphinx page in which one or more of your widgets simply does not appear at all.

We now elevate such Sphinx warnings to actual build-halting errors, so that the user is made aware of the issue.