* Stop throwing ParseError when throwOnError is false
`render`, `renderToString`, etc. now catch ParseError and render it to the
raw LaTeX (with proper escaping) and a hover title with the error.
Along the way:
* Use new `katex.__renderToDomTree` in katex-spec's `_getBuilt`.
(This was necessary to get the new error handling in `_getBuilt`.)
* Fix jest results which must always be functions, not strings.
* fix lint
* Fix flow error
Leave error type unspecified, as we check it with instanceof.
* Update katex-spec.js
