Add healthz to readme, add error case for set_not_found

README.md

@@ -391,11 +391,15 @@ pict-rs offers the following endpoints:
     GET /image/process.jpg?src=asdf.png&thumbnail=256&blur=3.0
     ```
     which would create a 256x256px JPEG thumbnail and blur it
-- `GET /image/process_backgrounded.{ext}?src={file}&...` queue transformations to be applied to a given file. This accepts the same arguments as the `process.{ext}` endpoint, but does not wait for the processing to complete.
+- `GET /image/process_backgrounded.{ext}?src={file}&...` queue transformations to be applied to a
+    given file. This accepts the same arguments as the `process.{ext}` endpoint, but does not wait
+    for the processing to complete.
 - `GET /image/details/process.{ext}?src={file}&...` for getting the details of a processed image.
     The returned JSON is the same format as listed for the full-resolution details endpoint.
 - `DELETE /image/delete/{delete_token}/{file}` or `GET /image/delete/{delete_token}/{file}` to
     delete a file, where `delete_token` and `file` are from the `/image` endpoint's JSON
+- `GET /healthz` Check the health of the pict-rs server. This will check that the `sled` embedded
+    database is functional and that the configured store is accessible
 
 
 The following endpoints are protected by an API key via the `X-Api-Token` header, and are disabled
@@ -446,6 +450,14 @@ A secure API key can be generated by any password generator.
     }
     ```
 
+    In the event pict-rs can't find the provided alias, it will return a 400 Bad Request with the
+    following json:
+    ```json
+    {
+        "msg": "No hash associated with provided alias"
+    }
+    ```
+
 Additionally, all endpoints support setting deadlines, after which the request will cease
 processing. To enable deadlines for your requests, you can set the `X-Request-Deadline` header to an
 i128 value representing the number of nanoseconds since the UNIX Epoch. A simple way to calculate