Improve Katex error handling and fix handling of large expressions

* Make throwOnError=true the new default
* Handle JS errors as part of the RPC request/response flow
* Return a new Result type with .Err on it

This enables constructs on the form:

```handlebars
{{ with transform.ToMath "c = \\foo{a^2 + b^2}" }}
	{{ with .Err }}
	 	{{ warnf "error: %s" . }}
	{{ else }}
		{{ . }}
	{{ end }}
{{ end }}
```

Note that the new `Result` type behaves like `template.HTML` (or a string if needed) when printed, but it will panic if in a error state.

Closes #12748

internal/warpc/js/renderkatex.js

@@ -5,7 +5,14 @@ const render = function (input) {
 	const data = input.data;
 	const expression = data.expression;
 	const options = data.options;
-	writeOutput({ header: input.header, data: { output: katex.renderToString(expression, options) } });
+	const header = input.header;
+	try {
+		const output = katex.renderToString(expression, options);
+		writeOutput({ header: header, data: { output: output } });
+	} catch (e) {
+		header.err = e.message;
+		writeOutput({ header: header });
+	}
 };
 
 readInput(render);