Bug 1568946 - Add better errors when addon install policy fails. r=aswan

Differential Revision: https://phabricator.services.mozilla.com/D39398

--HG--
extra : moz-landing-system : lando

browser/components/enterprisepolicies/Policies.jsm

@@ -1586,12 +1586,20 @@ function installAddonFromURL(url, extensionID) {
       },
       onDownloadFailed: () => {
         install.removeListener(listener);
-        log.error(`Download failed - ${url}`);
+        log.error(
+          `Download failed - ${AddonManager.errorToString(
+            install.error
+          )} - ${url}`
+        );
         clearRunOnceModification("extensionsInstall");
       },
       onInstallFailed: () => {
         install.removeListener(listener);
-        log.error(`Installation failed - ${url}`);
+        log.error(
+          `Installation failed - ${AddonManager.errorToString(
+            install.error
+          )} - {url}`
+        );
       },
       onInstallEnded: () => {
         install.removeListener(listener);