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