AgamaProposal: fix bug when partitions are searched but not managed

[ancorgs]

Problem

When a drive was configured with an empty search for partitions like this (in which there is no configuration about deleting or resizing the found partitions)...

{
  "storage": {
    "drives": [
      {
        "partitions": [
          { "search": "*" },
          { "filesystem": { "path": "/" } }
        ]
      }
    ]
  } 
} 

...all the matched pre-existing partitions are considered as candidates to be shrunk.

Solution

With this fix, partitions with no actions do not generate any SpaceMaker action.

Bonus

AgamaProposal now returns an exception of type Y2Storage::NoDiskSpaceError when the SpaceMaker fails to find space. That was the documented behavior, but a generic Y2Storage::Error was returned instead.

Testing

• Added a new unit test
• Tested manually