Put healthcheck errors in app logs

src/apps.js

@@ -61,6 +61,8 @@ exports = module.exports = {
     getLogPaths,
     getLogs,
 
+    appendLogLine,
+
     getCertificate,
 
     start,
@@ -2013,6 +2015,16 @@ async function getLogs(app, options) {
     return transformStream;
 }
 
+// never fails just prints error
+async function appendLogLine(app, line) {
+    assert.strictEqual(typeof app, 'object');
+    assert.strictEqual(typeof line, 'string');
+
+    const logFilePath = path.join(paths.LOG_DIR, app.id, 'app.log');
+
+    if (!safe.fs.appendFileSync(logFilePath, line)) console.error(`Could not append log line for app ${app.id} at ${logFilePath}: ${safe.error.message}`);
+}
+
 async function getCertificate(subdomain, domain) {
     assert.strictEqual(typeof subdomain, 'string');
     assert.strictEqual(typeof domain, 'string');