Validate scan shape on the server

The scan API view now uses a schema with validation
constraints to enforce a minimum size of 1 in any dimension.
This requires better error handling in the app, which I've added.
It also needs a fix in LabThings so the error propagates correctly

openflexure_microscope/api/app.py

@@ -23,7 +23,7 @@ import os
 from datetime import datetime
 
 import pkg_resources
-from flask import abort, send_file
+from flask import abort, send_file, jsonify
 from flask_cors import CORS, cross_origin
 from labthings import create_app
 from labthings.extensions import find_extensions
@@ -105,6 +105,19 @@ app, labthing = create_app(
 # Enable CORS for some routes outside of LabThings
 cors: CORS = CORS(app)
 
+# Enable correct handling of Marshmallow/Webargs validation errors
+# Return validation errors as JSON 
+# (see  https://webargs.readthedocs.io/en/latest/framework_support.html)
+@app.errorhandler(422)
+@app.errorhandler(400)
+def handle_error(err):
+    headers = err.data.get("headers", None)
+    messages = err.data.get("messages", ["Invalid request."])
+    if headers:
+        return jsonify({"errors": messages}), err.code, headers
+    else:
+        return jsonify({"errors": messages}), err.code
+
 # Use custom JSON encoder
 labthing.json_encoder = JSONEncoder
 app.json_encoder = JSONEncoder

openflexure_microscope/api/default_extensions/scan.py

@@ -2,6 +2,7 @@ import datetime
 import logging
 import time
 import uuid
+import marshmallow
 from functools import reduce
 from typing import Dict, List, Optional, Tuple
 
@@ -361,7 +362,7 @@ class ScanExtension(BaseExtension):
 
 class TileScanArgs(FullCaptureArgs):
     namemode = fields.String(missing="coordinates", example="coordinates")
-    grid = fields.List(fields.Integer, missing=[3, 3, 3], example=[3, 3, 3])
+    grid = fields.List(fields.Integer(validate=marshmallow.validate.Range(min=1)), missing=[3, 3, 3], example=[3, 3, 3], )
     style = fields.String(missing="raster")
     autofocus_dz = fields.Integer(missing=50)
     fast_autofocus = fields.Boolean(missing=False)