astakosclient: Human readable details message

In case of QuotaLimit exception, fill 'details' with a human readable
message and put the actual JSON response in the 'response' variable.
Synnefo UI shows the 'details' field of an exception to the user so it
has to be human readable.
astakosclient: Fallback to json module

If 'simplejson' module is not available fallback to 'json'.