trivial: a few improvements to in-line doc and code, and renaming of test/test_immutable_checker.py to test/test_immutable.py

That file currently tests checker and verifier and repairer, and will soon also test downloader.

src/allmydata/immutable/download.py

@@ -802,7 +802,7 @@ class FileDownloader(log.PrefixingLogMixin):
             self._results.timings["peer_selection"] = now - self._started
 
         if len(self._share_buckets) < self._uri.needed_shares:
-            raise NotEnoughSharesError
+            raise NotEnoughSharesError(len(self._share_buckets), self._uri.needed_shares)
 
         #for s in self._share_vbuckets.values():
         #    for vb in s:

src/allmydata/storage.py

@@ -344,6 +344,9 @@ class BucketReader(Referenceable):
         self.storage_index = storage_index
         self.shnum = shnum
 
+    def __repr__(self):
+        return "<%s %s %s>" % (self.__class__.__name__, base32.b2a_l(self.storage_index[:8], 60), self.shnum)
+
     def remote_read(self, offset, length):
         start = time.time()
         data = self._share_file.read_share_data(offset, length)

src/allmydata/test/test_immutable.py

@@ -288,7 +288,7 @@ class Test(ShareManglingMixin, unittest.TestCase):
         # download it is more to test this test code than to test the Tahoe code...
         def _then_delete_8(unused=None):
             self.replace_shares(stash[0], storage_index=self.uri.storage_index)
-            for sharenum in range(2, 10):
+            for i in range(8):
                 self._delete_a_share()
         d.addCallback(_then_delete_8)