IMPROVE: Clearer error message for unconfigured computers (#4670)

Previously the code just raised with a `NotExistent: No result was found`
message, but at the point of failure we can be more explicit about the
cause of the error (the missing configuration step). Now the error is
clearer and provides the solution to the problem.

aiida/orm/computers.py

@@ -600,7 +600,15 @@ def get_authinfo(self, user):
         """
         from . import authinfos
 
-        return authinfos.AuthInfo.objects(self.backend).get(dbcomputer_id=self.id, aiidauser_id=user.id)
+        try:
+            authinfo = authinfos.AuthInfo.objects(self.backend).get(dbcomputer_id=self.id, aiidauser_id=user.id)
+        except exceptions.NotExistent as exc:
+            raise exceptions.NotExistent(
+                f'Computer `{self.label}` (ID={self.id}) not configured for user `{user.get_short_name()}` '
+                f'(ID={user.id}) - use `verdi computer configure` first'
+            ) from exc
+
+        return authinfo
 
     def is_user_configured(self, user):
         """

tests/orm/test_computers.py

@@ -103,3 +103,19 @@ def test_configure_ssh_invalid(self):
 
         with self.assertRaises(ValueError):
             comp.configure(username='radames', invalid_auth_param='TEST')
+
+    def test_non_configure_error(self):
+        """Configure a computer for local transport and check it is configured."""
+        self.comp_builder.label = 'test_non_configure_error'
+        self.comp_builder.transport = 'local'
+        comp = self.comp_builder.new()
+        comp.store()
+
+        with self.assertRaises(exceptions.NotExistent) as exc:
+            comp.get_authinfo(self.user)
+
+        self.assertIn(str(comp.id), str(exc.exception))
+        self.assertIn(comp.label, str(exc.exception))
+        self.assertIn(self.user.get_short_name(), str(exc.exception))
+        self.assertIn(str(self.user.id), str(exc.exception))
+        self.assertIn('verdi computer configure', str(exc.exception))