Add explanation to new error logging.

src/allmydata/immutable/downloader/share.py

@@ -477,7 +477,7 @@ class Share(object):
                                                      str(f.value))
         self._rref.callRemote(
             "advise_corrupt_share", reason.encode("utf-8")
-        ).addErrback(log.err)
+        ).addErrback(log.err, "Error from remote call to advise_corrupt_share")
 
     def _satisfy_block_hash_tree(self, needed_hashes):
         o_bh = self.actual_offsets["block_hashes"]

src/allmydata/immutable/layout.py

@@ -254,7 +254,7 @@ class WriteBucketProxy(object):
         return d
 
     def abort(self):
-        self._rref.callRemote("abort").addErrback(log.err)
+        self._rref.callRemote("abort").addErrback(log.err, "Error from remote call to abort an immutable write bucket")
 
     def get_servername(self):
         return self._server.get_name()

src/allmydata/storage_client.py

@@ -1015,4 +1015,4 @@ class _StorageServer(object):
             storage_index,
             shnum,
             reason,
-        ).addErrback(log.err)
+        ).addErrback(log.err, "Error from remote call to advise_corrupt_share")