Improve Katex error handling

[bep]

• 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:

{{ 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