ParseTex should return an error value


### Discussed in https://github.com/Wyatt915/treeblood/discussions/18

<div type='discussions-op-text'>

<sup>Originally posted by **Wyatt915** July 12, 2025</sup>
Currently, any (nonfatal) parse errors produce an `<merror>` tag in the mathml tree without complaint. Would it be useful to know if nonfatal parsing or tokenization errors occurred by bubbling up an error value?</div>