(chore): add error string to handle_snapshot

ddapm_test_agent/agent.py

@@ -30,6 +30,7 @@
 from aiohttp import web
 from aiohttp.web import Request
 from aiohttp.web import middleware
+from aiohttp.web import HTTPException
 from msgpack.exceptions import ExtraData as MsgPackExtraDataException
 from multidict import CIMultiDict
 
@@ -107,6 +108,18 @@ async def session_token_middleware(request: Request, handler: _Handler) -> web.R
     return await handler(request)
 
 
+@middleware
+async def handle_exception_middleware(request: Request, handler: _Handler) -> web.Response:
+        """Turn exceptions into 400s with the reason from the exception."""
+        try:
+            response = await handler(request)
+            return response
+        except HTTPException:
+            raise
+        except Exception as e:
+            return web.HTTPBadRequest(reason=str(e))
+
+
 async def _forward_request(request_data: bytes, headers: Mapping[str, str], full_agent_url: str) -> ClientResponse:
     async with ClientSession() as session:
         async with session.post(
@@ -1035,6 +1048,7 @@ def make_app(
     app = web.Application(
         client_max_size=int(100e6),  # 100MB - arbitrary
         middlewares=[
+            handle_exception_middleware,  #type: ignore
             agent.check_failure_middleware,  # type: ignore
             agent.store_request_middleware,  # type: ignore
             agent.request_forwarder_middleware,  # type: ignore