Bug 1642150 - explain cause of exchange account setup failure. r=BenB

mail/components/accountcreation/content/emailWizard.js

@@ -2095,7 +2095,16 @@ EmailConfigWizard.prototype = {
         // Could be a wrong password, but there are 1000 other
         // reasons why this failed. Only the backend knows.
         // If we got no message, then something other than VerifyLogon failed.
-        self.showErrorMsg(e.message || e.toString());
+
+        let msg = e.message || e.toString();
+        // For an Exchange server, some known configurations can
+        // be disabled (per user or domain or server).
+        if (
+          configFilledIn.incomingAlternatives.some(i => i.type == "exchange")
+        ) {
+          msg = gStringsBundle.getString("exchange_config_unverifiable");
+        }
+        self.showErrorMsg(msg);
 
         // TODO use switchToMode(), see above
         // give user something to proceed after fixing

mail/locales/en-US/chrome/messenger/accountCreation.properties

@@ -59,6 +59,7 @@ probing_config=probing configuration…
 guessing_from_email=guessing configuration…
 config_details_found=Your configuration details have been found!
 config_unverifiable=Configuration could not be verified — is the username or password wrong?
+exchange_config_unverifiable=Configuration could not be verified. If your username and password are correct, it's likely that the server administrator has disabled the selected configuration for your account. Try selecting another protocol.
 incoming_found_specify_outgoing=Your incoming server configuration details have been found, please specify the sending hostname.
 outgoing_found_specify_incoming=Your outgoing server configuration details have been found, please specify the receiving hostname.
 please_enter_missing_hostnames=Could not guess settings — please enter missing hostnames.