Summary: Please note that the linter complained about the order of a few methods so I had to move them around.

Test Plan:
Check out some fixtures that use tex.jsx such as
`texified-code.jsx.fixture.js` and ensure everything renders properly.

Reviewers: #webpack, john

Reviewed By: #webpack, john

Subscribers: john, kevinb, davidflanagan

Differential Revision: https://phabricator.khanacademy.org/D49193

Summary:
Bugs CP-879 and CP-1742 are caused by a TypeError thrown by KaTeX.
We already fall back to MathJAX rendering when KaTeX throws a parse
error. But currently we rethrow other kinds of errors. This patch
changes that so we fallback to MathJAX for any kind of KaTeX error.

The underlying KaTeX error has been fixed upstream and will go away
when we upgrade webapp to use KaTeX 1.0. Since we've got the MathJAX
fallback, however, it seems worth using it in case other errors arise
in the future.

Test Plan:
Visit the following URL with this patch and verifies that it renders:
http://localhost:8081/math/k-8-grades/fr-v2-terminale-s/fr-v2-matrices-enseignement-de-spcialit/fr-v2-oprations-sur-les-matrices/a/multiplying-matrices-by-scalars?lang=fr

Reviewers: kevinb, michaelpolyak

Reviewed By: kevinb

Subscribers: tom

Differential Revision: https://phabricator.khanacademy.org/D50139