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/common.js

@@ -30,13 +30,23 @@ export function readInput(handle) {
 
 		currentLine = [...currentLine, ...buffer.subarray(0, bytesRead)];
 
+		// Check for newline. If not, we need to read more data.
+		if (!currentLine.includes(10)) {
+			continue;
+		}
+
 		// Split array into chunks by newline.
 		let i = 0;
 		for (let j = 0; i < currentLine.length; i++) {
 			if (currentLine[i] === 10) {
 				const chunk = currentLine.splice(j, i + 1);
 				const arr = new Uint8Array(chunk);
-				const json = JSON.parse(new TextDecoder().decode(arr));
+				let json;
+				try {
+					json = JSON.parse(new TextDecoder().decode(arr));
+				} catch (e) {
+					throw new Error(`Error parsing JSON '${new TextDecoder().decode(arr)}' from stdin: ${e.message}`);
+				}
 				handle(json);
 				j = i + 1;
 			}