Implement normal error message

Error handling request
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/aiohttp/web_protocol.py", line 418, in start
    resp = await task
  File "/usr/local/lib/python3.8/site-packages/aiohttp/web_app.py", line 458, in _handle
    resp = await handler(request)
  File "/document_service/prozorro_sale/document_service/api.py", line 37, in upload_document
    document_data = await request.app.storage.upload(data, scope)
  File "/document_service/prozorro_sale/document_service/storages/s3_storage.py", line 30, in upload
    content_type = data['Content-Type']
KeyError: 'Content-Type'