osbuild-mpp: fix PartitionTable.__getitem__() bail-out

The __getitem__() function incorrectly returns `None` if the
partition-type is not found. Make sure to always return IndexError if
the requested value is outside the supported range.

Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>

tools/osbuild-mpp

@@ -827,8 +827,7 @@ class PartitionTable:
             for part in self.partitions:
                 if part.id == key:
                     return part
-        else:
-            raise ValueError("unsupported type")
+        raise IndexError
 
     def write_to(self, target, sync=True):
         """Write the partition table to disk"""