fix: Check if homepage is not setup-wizard

The new desk changes sets the default homepage to 'workspace'. To avoid
problem in the future, check if the value is not setup-wizard.
Also throw an exception if the check passes, so it exits with error.
diff --git a/erpnext/setup/install.py b/erpnext/setup/install.py
index 74ff0ec..44f26bf 100644
--- a/erpnext/setup/install.py
+++ b/erpnext/setup/install.py
@@ -29,12 +29,12 @@
 
 
 def check_setup_wizard_not_completed():
-	if frappe.db.get_default('desktop:home_page') == 'desktop':
-		print()
-		print("ERPNext can only be installed on a fresh site where the setup wizard is not completed")
-		print("You can reinstall this site (after saving your data) using: bench --site [sitename] reinstall")
-		print()
-		return False
+	if frappe.db.get_default('desktop:home_page') != 'setup-wizard':
+		message = """Cannot install ERPNext.
+
+ERPNext can only be installed on a fresh site where the setup wizard is not completed.
+You can reinstall this site (after saving your data) using: bench --site [sitename] reinstall"""
+		frappe.throw(message, exc=frappe.IncompatibleApp)
 
 
 def set_single_defaults():