Error implementations (#232)

Solves #216 by adding a check on table open for existence of the
`.lance` file. Does not check for it for remote connections.

python/tests/test_db.py

@@ -123,6 +123,15 @@ def test_delete_table(tmp_path):
     assert db.table_names() == ["test"]
 
 
+def test_empty_or_nonexistent_table(tmp_path):
+    db = lancedb.connect(tmp_path)
+    with pytest.raises(Exception):
+        db.create_table("test_with_no_data")
+
+    with pytest.raises(Exception):
+        db.open_table("does_not_exist")
+
+
 def test_replace_index(tmp_path):
     db = lancedb.connect(uri=tmp_path)
     table = db.create_table(