Improve handling of errors in Rucio REST API calls

[ftorradeflot]

The Rucio jupyterlab extension is basically a passthrough between the frontend and the Rucio Server's REST API. The actions on the frontend are transformed in API requests sent to the extension backend which are then transformed and forwarded to Rucio's REST API.

In some cases, the backend may receive a 500 error which is not properly propagated to the frontend. For example:

• Add a metadata attribute to a file rucio set-metadata --did test:file1 --key x --value 1
• List the file: rucio -vv list-dids --filter 'type=all,x=1' test:* . Returns a 500 error with the message: "Filter keys used do not all belong to the same metadata plugin."
• When we do the same search through the extension, we don't get any error, just "No results found"