Downloading project produce a crash in the plugin when server in maitenance mode

When server is in maitenance mode and you try to download a project, the plugin crash

Traceback (most recent call last):

  File "/home/jozef/.local/share/QGIS/QGIS3/profiles/ws/python/plugins/Mergin/mergin/client.py", line 206, in _do_request
    return self.opener.open(request)

  File "/usr/lib/python3.10/urllib/request.py", line 525, in open
    response = meth(req, response)

  File "/usr/lib/python3.10/urllib/request.py", line 634, in http_response
    response = self.parent.error(

  File "/usr/lib/python3.10/urllib/request.py", line 563, in error

    return self._call_chain(*args)

  File "/usr/lib/python3.10/urllib/request.py", line 496, in _call_chain
    result = func(*args)

  File "/usr/lib/python3.10/urllib/request.py", line 643, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)

urllib.error.HTTPError: HTTP Error 503: Service Unavailable


During handling of the above exception, another exception occurred:


Traceback (most recent call last):

  File "/home/jozef/.local/share/QGIS/QGIS3/profiles/ws/python/plugins/Mergin/sync_dialog.py", line 113, in download_start_internal
    self.job = download_project_async(self.mergin_client, self.project_name, self.target_dir)

  File "/home/jozef/.local/share/QGIS/QGIS3/profiles/ws/python/plugins/Mergin/mergin/client_pull.py", line 143, in download_project_async
    latest_proj_info = mc.project_info(project_path)

  File "/home/jozef/.local/share/QGIS/QGIS3/profiles/ws/python/plugins/Mergin/mergin/client.py", line 692, in project_info
    resp = self.get("/v1/project/{}".format(project_path_or_id), params)

  File "/home/jozef/.local/share/QGIS/QGIS3/profiles/ws/python/plugins/Mergin/mergin/client.py", line 230, in get
    return self._do_request(request)

  File "/home/jozef/.local/share/QGIS/QGIS3/profiles/ws/python/plugins/Mergin/mergin/client.py", line 195, in wrapper
    return f(self, *args)

  File "/home/jozef/.local/share/QGIS/QGIS3/profiles/ws/python/plugins/Mergin/mergin/client.py", line 220, in _do_request
    raise ClientError(error_msg)

Mergin.mergin.common.ClientError: HTTP Error: 503 Service Unavailable
URL: https://app.dev.merginmaps.com/v1/project/tme1/aextractor3
Method: GET
Detail: { "status" : "503", "title" : "Service unavailable", "detail" : "The site is undergoing scheduled maintenance." }

https://github.com/MerginMaps/qgis-plugin/assets/83582181/4dfb5f04-ca7b-4856-9aa9-55fb044c2b44

Start in mergin/client.py at _do_request and project_info, then follow the download flow through mergin/client_pull.py and sync_dialog.py:113. Reproduce the 503 maintenance response shown in the traceback and trace how it reaches the download UI. Done means downloading during maintenance no longer crashes and the failure is presented appropriately to the user.