Document which failure a database that will not open raises

Four passages said every missing database raises `FileNotFoundError` and that
errors show locations. A database named in `lancedb.databases` raises
`SourceUnavailableError` instead, naming the database and not its location,
which is the point of naming them. A path you gave keeps `FileNotFoundError` and
still shows the path.

CHANGELOG.md

@@ -12,7 +12,8 @@
   disabled. Operations that need one database raise `AmbiguousDatabaseError`, an
   unknown name raises `UnknownDatabaseError`, and a cited chunk ID retrieved from
   or previously cited from more than one selected database raises
-  `AmbiguousCitationError`.
+  `AmbiguousCitationError`. An unavailable configured database raises
+  `SourceUnavailableError`, which names the database and not its location.
 - `haiku-rag search`, `ask`, `analyze` and `chat` cover a configured set.
   Commands that access one database select it with `--db-name NAME` or
   `--db PATH`.

docs/configuration/storage.md

@@ -115,7 +115,7 @@ async with HaikuRAG(create=True) as client:
 
 The [default location](index.md#configuration-file-locations) is platform-specific (e.g., `~/Library/Application Support/haiku.rag/` on macOS).
 
-Operations on non-existent databases raise `FileNotFoundError`. This prevents accidental database creation from typos or misconfigured paths.
+Opening a nonexistent unnamed local database raises `FileNotFoundError`, naming its path. This prevents accidental database creation from typos or misconfigured paths. A database named in `lancedb.databases` raises `SourceUnavailableError` instead, naming the database and not its location.
 
 ## Remote Storage
 
@@ -216,8 +216,12 @@ lancedb:
 A location can be a URI or local path. `databases` and `uri` are mutually
 exclusive.
 
-Results, documents, and citations use the configured name as `source`. Commands
-such as `info` and path-related errors still show locations.
+Results, documents, and citations use the configured name as `source`. An
+unavailable configured database raises `SourceUnavailableError`, which names the
+database and not its location, so a location never travels in an error a consumer
+might render or log. A migration, configuration or read-only failure keeps its
+own type, with the database named in the message. Commands that report on a
+database, such as `info`, still show where it is.
 
 Searches spanning multiple databases identify each result with a model-facing
 `Collection:` line. Searches over one database omit it. Structured `source`
@@ -310,8 +314,8 @@ Image queries are vector-only and skip the reranker: there is no query text to
 score a document against, so candidates keep their vector ranking and fusion
 ranks by position.
 
-If any selected database fails to open, the operation fails and identifies that
-database.
+If a selected database is unavailable, the operation fails with
+`SourceUnavailableError`, which names that database.
 
 ### Python Operations
 

docs/python.md

@@ -31,7 +31,7 @@ else, since draining the background vacuum and releasing the embedder and
 reranker are awaitable; it refuses a client covering several.
 
 !!! note
-    Databases must be explicitly created with `create=True` or via `haiku-rag init` before use. Operations on non-existent databases will raise `FileNotFoundError`.
+    Databases must be explicitly created with `create=True` or via `haiku-rag init` before use. Opening a nonexistent unnamed local database raises `FileNotFoundError`, naming its path; one named in `lancedb.databases` raises `SourceUnavailableError`, which names the database rather than its location.
 
 !!! note
     Read-only mode is useful for safely accessing databases without risk of modification. It blocks all write operations and downgrades an embedding provider/name mismatch to a warning instead of raising `ConfigMismatchError`.