Display a message in the dashboard if a capture cannot be taken

Regarding

+ * Return the value for status column based on status and last checked
+ */
+function _amber_status_column_value($status, $last_checked = "2 ") {
+  $value = $status ? 'Up' : 'Down';
+  if ($last_checked != NULL && empty($last_checked)) {
+    $value = "";
+  }
+  return $value;
+}

Two comments:
* I don't think we need to provide a default value for $last_checked.
* The logic for checking $last_checked behaves differently if the value is NULL and if it's an empty string - this is confusing.

I think this function can be similar to the Wordpress one:

+ * Return the value for status column based on status and last checked
*/
function _amber_status_column_value($status, $last_checked) {
 if (empty($last_checked)) {
   return "";
 } else {
   return $status ? 'Up' : 'Down';
}

If you agree, please preroll the patch and resubmit.

The change to the _amber_status_column_value() function looks good.

One more change before applying this patch - PHP close tags have been added to a few of the files. This is not desired - see https://www.drupal.org/coding-standards#phptags

Committed a change that builds upon the previous patches, but adds some more features:

* Adds an extra column ('message') to the amber_cache table which allows us to track errors associated with the caching process for a specific backend, rather than for the checking process, which is backend-agnostic. For example, if you are caching to Perma and your local disk, you might receive an error from Perma, but still be able to save locally. This column allows us to keep the error message associated with the backend, and display appropriately on the frontend

* Updated the AmberFetcher, PermaFetcher, and InternetArchiveFetcher to provide more information in case of errors (e.g. url, provider id) rather than just an error message string - which allows us to save an error message and associate it with a specific cache attempt with a specific backend