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;
 			}

internal/warpc/js/greet.bundle.js

@@ -1,2 +1,2 @@
-(()=>{function i(r){let e=[],a=new Uint8Array(1024);for(;;){let n=0;try{n=Javy.IO.readSync(0,a)}catch(o){if(o.message.includes("os error 29"))break;throw new Error("Error reading from stdin")}if(n<0)throw new Error("Error reading from stdin");if(n===0)break;e=[...e,...a.subarray(0,n)];let t=0;for(let o=0;t<e.length;t++)if(e[t]===10){let f=e.splice(o,t+1),s=new Uint8Array(f),u=JSON.parse(new TextDecoder().decode(s));r(u),o=t+1}e=e.slice(t)}}function d(r){let c=new TextEncoder().encode(JSON.stringify(r)+`
-`),e=new Uint8Array(c);Javy.IO.writeSync(1,e)}var l=function(r){d({header:r.header,data:{greeting:"Hello "+r.data.name+"!"}})};console.log("Greet module loaded");i(l);})();
+(()=>{function s(r){let e=[],c=new Uint8Array(1024);for(;;){let n=0;try{n=Javy.IO.readSync(0,c)}catch(o){if(o.message.includes("os error 29"))break;throw new Error("Error reading from stdin")}if(n<0)throw new Error("Error reading from stdin");if(n===0)break;if(e=[...e,...c.subarray(0,n)],!e.includes(10))continue;let t=0;for(let o=0;t<e.length;t++)if(e[t]===10){let u=e.splice(o,t+1),d=new Uint8Array(u),a;try{a=JSON.parse(new TextDecoder().decode(d))}catch(l){throw new Error(`Error parsing JSON '${new TextDecoder().decode(d)}' from stdin: ${l.message}`)}r(a),o=t+1}e=e.slice(t)}}function f(r){let i=new TextEncoder().encode(JSON.stringify(r)+`
+`),e=new Uint8Array(i);Javy.IO.writeSync(1,e)}var w=function(r){f({header:r.header,data:{greeting:"Hello "+r.data.name+"!"}})};console.log("Greet module loaded");s(w);})();

internal/warpc/js/renderkatex.js

@@ -5,7 +5,14 @@ const render = function (input) {
 	const data = input.data;
 	const expression = data.expression;
 	const options = data.options;
-	writeOutput({ header: input.header, data: { output: katex.renderToString(expression, options) } });
+	const header = input.header;
+	try {
+		const output = katex.renderToString(expression, options);
+		writeOutput({ header: header, data: { output: output } });
+	} catch (e) {
+		header.err = e.message;
+		writeOutput({ header: header });
+	}
 };
 
 readInput(render);