Generalize ingestion status to a string, and mark/skip failed ingestions

[gvanrossum-ms]

This changes the schema for IngestedSources, adding a text column 'status' that can describe whether indexing of a source_id succeeded, failed, or resulted in some other status. Well-known values are 'ingested', 'failed', but using mark_source_ingested() it can be set to any other string value, and the status can be retrieved using get_source_status(). The existing is_source_ingested() returns True only if the status field has the exact value 'ingested'.

The behavior of add_messages_with_indexing() changes subtly: it now sets the status to 'failed' before attempting any work; it sets it to 'ingested' once it is done indexing.

To alter a precious existing database, do this using sqlite3:

ALTER TABLE IngestedSources
ADD COLUMN status TEXT NOT NULL DEFAULT 'ingested';

That sets the status for every row present to 'ingested', matching the original behavior.