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

common/types/types.go

@@ -120,3 +120,27 @@ var InvocationCounter atomic.Int64
 func NewBool(b bool) *bool {
 	return &b
 }
+
+// PrintableValueProvider is implemented by types that can provide a printable value.
+type PrintableValueProvider interface {
+	PrintableValue() any
+}
+
+var _ PrintableValueProvider = Result[any]{}
+
+// Result is a generic result type.
+type Result[T any] struct {
+	// The result value.
+	Value T
+
+	// The error value.
+	Err error
+}
+
+// PrintableValue returns the value or panics if there is an error.
+func (r Result[T]) PrintableValue() any {
+	if r.Err != nil {
+		panic(r.Err)
+	}
+	return r.Value
+}