log and ignore systems (in the snapd sense) we cannot install

These errors are really errors in how the ISO is constructed, so logging
and ignoring seems more appropriate, compared to say encryption being
unavailable because the TPM is in lockout mode.

subiquity/server/controllers/filesystem.py

@@ -261,6 +261,19 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
         return system
 
     def info_for_system(self, name: str, label: str, system: SystemDetails):
+        if len(system.volumes) > 1:
+            log.error(
+                "Skipping uninstallable system: %s",
+                system_multiple_volumes_text)
+            return None
+
+        [volume] = system.volumes.values()
+        if volume.schema != 'gpt':
+            log.error(
+                "Skipping uninstallable system: %s",
+                system_non_gpt_text)
+            return None
+
         info = VariationInfo(
             name=name,
             label=label,
@@ -268,15 +281,6 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
             system=system,
             )
 
-        if len(system.volumes) > 1:
-            info.error = system_multiple_volumes_text
-            return info
-
-        [volume] = system.volumes.values()
-        if volume.schema != 'gpt':
-            info.error = system_non_gpt_text
-            return info
-
         se = system.storage_encryption
         if se.support == StorageEncryptionSupport.DEFECTIVE:
             info.error = system_defective_encryption_text.format(
@@ -321,8 +325,9 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
                 system = await self._get_system(name, label)
             log.debug("got system %s for variation %s", system, name)
             if system is not None and len(system.volumes) > 0:
-                self._variation_info[name] = self.info_for_system(
-                    name, label, system)
+                info = self.info_for_system(name, label, system)
+                if info is not None:
+                    self._variation_info[name] = info
             else:
                 # This calculation is pretty much a hack and we should
                 # actually think about it at some point (like: maybe the