astakosclient: Fallback to Python's json package

If simplejson is not installed, fallback to python's default 'json'
package. This is needed because 'simplejson' is not available in some
distributions (i.e. CentOs 7).
pithos: Change quota error handling

Pithos backend used to catch errors generated by the AstakosClient
and in those cases would return 413 (Request Entity too Large).
However, quota limit failures are not the only failures can occur.
Therefore, 413 responses are not always appropriate.
To fix this problem, let the respective methods fail with an
AstakosClientException which is handled by the common api decorator.