If you don't mind importing the json module, then the best way to handle it is through json.JSONDecodeError (or json.decoder.JSONDecodeError as they are the same) as using default errors like ValueError could catch also other exceptions not necessarily connected to the json decode one.
from json.decoder import JSONDecodeError
try:
qByUser = byUsrUrlObj.read()
qUserData = json.loads(qByUser).decode('utf-8')
questionSubjs = qUserData["all"]["questions"]
except JSONDecodeError as e:
# do whatever you want
//EDIT (Oct 2020):
As @Jacob Lee noted in the comment, there could be the basic common TypeError raised when the JSON object is not a str, bytes, or bytearray. Your question is about JSONDecodeError, but still it is worth mentioning here as a note; to handle also this situation, but differentiate between different issues, the following could be used:
from json.decoder import JSONDecodeError
try:
qByUser = byUsrUrlObj.read()
qUserData = json.loads(qByUser).decode('utf-8')
questionSubjs = qUserData["all"]["questions"]
except JSONDecodeError as e:
# do whatever you want
except TypeError as e:
# do whatever you want in this case