[DocsUI, Search]: Follow-up to ag/34419958

ag/34419958 moved handling exceptions to search loader and folder
loader. SearchLoader howerver never sets the cursor on exceptions. This
causes the future to wait for ever to be completed. This fixes the bug,
adds a test to confirm it.

Bug: 427555916
Test: atest DocumentsUIGoogleTests
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: Icdcce7cc85b2e8cd4cb5187a4ec2175a76bec29e