Handle KaTeX warnings (#13760)

Co-authored-by: Joe Mooring <joe.mooring@veriphor.com>

Fixes #13735

internal/warpc/js/renderkatex.js

@@ -7,6 +7,16 @@ const render = function (input) {
 	const expression = data.expression;
 	const options = data.options;
 	const header = input.header;
+	header.warnings = [];
+
+	if (options.strict == 'warn') {
+		// By default, KaTeX will write to console.warn, that's a little hard to handle.
+		options.strict = (errorCode, errorMsg) => {
+			header.warnings.push(
+				`katex: LaTeX-incompatible input and strict mode is set to 'warn': ${errorMsg} [${errorCode}]`,
+			);
+		};
+	}
 	// Any error thrown here will be caught by the common.js readInput function.
 	const output = katex.renderToString(expression, options);
 	writeOutput({ header: header, data: { output: output } });