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

tpl/internal/go_templates/htmltemplate/content.go

@@ -29,7 +29,8 @@ const (
 
 // indirect returns the value, after dereferencing as many times
 // as necessary to reach the base type (or nil).
-func indirect(a any) any {
+// Signature modified by Hugo. TODO(bep) script this.
+func doIndirect(a any) any {
 	if a == nil {
 		return nil
 	}