Fix the build by adding an argument to catch statement

Summary:
Turns out D22722736 (36de6f1) broke the OSS build: apparently `catch` without arguments is a [fairly new feature](https://stackoverflow.com/questions/21624456/can-i-use-a-try-catch-in-javascript-without-specifying-the-catch-argument-identi) that UglifyJS doesn't understand. Whoops. It threw:

```
ERROR: Unexpected token: punc «{», expected: punc «(»
```

Go figure.

It seems unlikey that it's possible, but it'd be cool if the TravisCI build status could block lands in www. There's no way to get signal about this until after the fact. /:

Reviewed By: creedarky

Differential Revision: D22771035

fbshipit-source-id: cd61c6a3ca2aab534c99939a4fb7f29a8863c2ea

Author: mrkev

src/model/encoding/convertFromHTMLToContentBlocks.js

@@ -176,7 +176,10 @@ const isValidAnchor = (node: Node) => {
     // Just checking whether we can actually create a URI
     const _ = new URI(anchorNode.href);
     return true;
-  } catch {
+    // We need our catch statements to have arguments, else
+    // UglifyJS (which we use for our OSS builds) will crash.
+    // eslint-disable-next-line fb-www/no-unused-catch-bindings
+  } catch (_) {
     return false;
   }
 };