fix(route): Handle error on parameter format for route /things

If parameter `format` is not in ['array', 'collection'] returns an
appropriate error message.

tdd/__init__.py

@@ -18,7 +18,7 @@
 import time
 from threading import Thread
 import click
-from flask import Flask, request, Response, stream_with_context
+from flask import Flask, request, Response, stream_with_context, HTTPBadRequest
 import json
 from flask.cli import FlaskGroup
 import json_merge_patch
@@ -109,9 +109,7 @@ def create_app():
     for entry_point in entry_points(group="tdd_api.plugins.blueprints"):
         try:
             app.register_blueprint(entry_point.load())
-            print(
-                f"Imported {entry_point.value} blueprint"
-            )
+            print(f"Imported {entry_point.value} blueprint")
         except Exception as exc:
             print(f"ERROR ({entry_point.name}): {exc}")
             print(
@@ -121,9 +119,7 @@ def create_app():
     for entry_point in entry_points(group="tdd_api.plugins.transformers"):
         try:
             TD_TRANSFORMERS.append(entry_point.load())
-            print(
-                f"Imported {entry_point.value} transformer"
-            )
+            print(f"Imported {entry_point.value} transformer")
         except Exception as exc:
             print(f"ERROR ({entry_point.name}): {exc}")
             print(
@@ -344,6 +340,9 @@ def generate():
                 f' etag="{get_collection_etag()}"'
             )
             return response
+        raise HTTPBadRequest(
+            "'format' parameter is not recognized. Must be in 'array' or 'collection' if defined."
+        )
 
     @app.route("/things/<id>", methods=["GET"])
     def describe_td(id):