Fallback to MathJAX on any Katex error, not just parse errors

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

Author: David Flanagan

js/tex.jsx

@@ -241,11 +241,24 @@ const TeX = createReactClass({
                 ),
             };
         } catch (e) {
-            /* jshint -W103 */
-            if (e.__proto__ !== katex.ParseError.prototype) {
-                /* jshint +W103 */
-                throw e;
-            }
+            // By catching the exception here and returning null
+            // we will fall back to asyncronously rendering with
+            // MathJAX.
+            //
+            // NOTE: formerly we only returned null if the error
+            // was a parse error from Katex and re-threw any other errors.
+            // But https://khanacademy.atlassian.net/browse/CP-879 and
+            // https://khanacademy.atlassian.net/browse/CP-1742 were caused
+            // by regular TypeError exceptions in Katex, so we might as
+            // well fall back to MathJAX in that case as well. (The Katex
+            // bug is fixed in the latest version and will stop happening
+            // when we upgrade webapp to use Katex 1.0.)
+            //
+            // TODO: We could use Raven.captureMessage() to send a message
+            // to Sentry when these errors occur if we want to get serious
+            // about eliminating them. Such a message should include
+            // window.location, props.children (the string of katex source)
+            // and the error itself.
             return null;
         }
     },