ttrss: Don't show app in enabled list of apps if install fails

Closes: #1615.

Tests:

- Throw an unconditional exception in enable_api_access() method. Reproduce the
problem described by installing ttrss app. Installation fails but ttrss shows up
in enabled list.

- Apply the patch. Try to install ttrss. When it fails, the app is not in the
enabled list.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/ttrss/__init__.py

@@ -94,7 +94,6 @@ class TTRSSApp(app_module.App):
 
     def enable(self):
         """Enable components and API access."""
-        super().enable()
         privileged.enable_api_access()
 
         # Try to set the domain to one of the available TLS domains
@@ -104,6 +103,8 @@ class TTRSSApp(app_module.App):
             domain = next(names.get_available_tls_domains(), None)
             privileged.set_domain(domain)
 
+        super().enable()
+
     def setup(self, old_version):
         """Install and configure the app."""
         privileged.pre_setup()