Change non-fatal directive parsing errors to warnings

[rowanc1]

From executablebooks/MyST-Parser#682

For non-fatal errors, such as;
faulty options syntax, unknown option keys, and invalid option values,
a warning is raised, but the directive is still run (without the erroneous options).
The warning is given the myst.directive_parse type, which can be suppressed.

We should adopt a similar approach here.