Stop throwing ParseError when throwOnError is false (#1169)

* 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

test/__snapshots__/katex-spec.js.snap

@@ -1,5 +1,39 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`A parser that does not throw on unsupported commands should build katex-error span for other type of KaTeX error 1`] = `
+{
+  "attributes": {
+    "style": "color:#933",
+    "title": "ParseError: KaTeX parse error: Double superscript at position 4: 2^2^̲2"
+  },
+  "children": [
+    {
+      "classes": [
+      ],
+      "depth": 0,
+      "height": 0,
+      "italic": 0,
+      "maxFontSize": 0,
+      "skew": 0,
+      "style": {
+      },
+      "value": "2^2^2",
+      "width": 0
+    }
+  ],
+  "classes": [
+    "katex-error"
+  ],
+  "depth": 0,
+  "height": 0,
+  "maxFontSize": 0,
+  "style": {
+  }
+}
+`;
+
+exports[`A parser that does not throw on unsupported commands should properly escape LaTeX in errors 1`] = `"<span class=\\"katex-error\\" title=\\"ParseError: KaTeX parse error: Expected group after &#x27;^&#x27; at position 2: 2^̲&amp;&quot;&lt;&gt;\\" style=\\"color:#933\\">2^&amp;&quot;&lt;&gt;</span>"`;
+
 exports[`An implicit group parser within optional groups should work style commands \\sqrt[\\textstyle 3]{x} 1`] = `
 [
   {