gaps: use ValueError for split

Callers may want to catch that.

subiquity/common/filesystem/gaps.py

@@ -47,7 +47,7 @@ class Gap:
         the supplied size.  If size is equal to the gap size, the second gap is
         None.  The original gap is unmodified."""
         if size > self.size:
-            raise Exception('requested size larger than gap')
+            raise ValueError('requested size larger than gap')
         if size == self.size:
             return (self, None)
         first_gap = Gap(device=self.device,

subiquity/common/filesystem/tests/test_gaps.py

@@ -45,7 +45,7 @@ class TestSplitGap(unittest.TestCase):
 
     def test_too_big(self):
         [gap] = gaps.parts_and_gaps(make_disk())
-        with self.assertRaises(Exception):
+        with self.assertRaises(ValueError):
             gap.split(gap.size + MiB)
 
     def test_split(self):