Merge branch 'aevri/cascache_nits' into 'master'

cascache.py: pick some nits

See merge request BuildStream/buildstream!1433
diff --git a/src/buildstream/_cas/cascache.py b/src/buildstream/_cas/cascache.py
index 434e716..53f6029 100644
--- a/src/buildstream/_cas/cascache.py
+++ b/src/buildstream/_cas/cascache.py
@@ -246,7 +246,7 @@
                 raise CASCacheError("Failed to pull ref {}: {}".format(ref, e)) from e
             else:
                 return False
-        except BlobNotFound as e:
+        except BlobNotFound:
             return False
 
     # pull_tree():
@@ -367,6 +367,9 @@
         # Exactly one of the two parameters has to be specified
         assert (path is None) != (buffer is None)
 
+        # If we're linking directly, then path must be specified.
+        assert (not link_directly) or (link_directly and path)
+
         if digest is None:
             digest = remote_execution_pb2.Digest()
 
@@ -400,7 +403,7 @@
                 os.makedirs(os.path.dirname(objpath), exist_ok=True)
                 os.link(tmp.name, objpath)
 
-        except FileExistsError as e:
+        except FileExistsError:
             # We can ignore the failed link() if the object is already in the repo.
             pass