update readme

monitor-sheet/Monitor.gs

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+
+const ERROR_MESSAGE = "NOT_FOUND"
+const SHEET_TO_MONITOR = "your-sheet-name"
+const ALERT = "Custom alert message for this error"
+
+function main() {
+  console.time("full");
+  {
+    let sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName(SHEET_TO_MONITOR);
+    assert(!findInSheet(sheet, ERROR_MESSAGE), ALERT)
+  }
+  console.timeEnd("full");
+}
+
+function findInSheet(sheet, SEARCH_FOR) {
+  let rows = sheet.getDataRange().getValues();
+  for (const row of rows) {
+    for (const cell of row) {
+      if (cell == SEARCH_FOR) {
+        console.log(`"${SEARCH_FOR}" FOUND in "${sheet.getSheetName()}"`)
+        return true;
+      }
+    }
+  }
+  console.log(`"${SEARCH_FOR}" NOT FOUND in "${sheet.getSheetName()}"`)
+  return false;
+}
+
+function assert(condition, message) {
+  if (!condition) {
+    console.error(message);
+    throw message;
+  }
+}

monitor-sheet/README.md

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+### Goal
+This script can be added to any sheet and then used in conjunction with the Timed executions + immediate email notification to have an alert in case the content of a sheet shows a given error message.
+
+### Features:
+* monitors a sheet for any string value
+* throws an error when a given string is found/not-found
+
+### How
+Use the gsheets formulas to detect errors/bugs and write something like `NOT_FOUND` when an error occurs (see [IFERROR](https://support.google.com/docs/answer/3093304?hl=en) method), then set up recurrent triggers and automated alerts for the sheet being monitored and you will get an email if something breaks in the sheet.