* 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
