Handle JSON decoding errors
Handle JSON decoding errors

JSON decoding errors raise ValueError instead of IOError. In such
cases, the best is to just act as if there was no cache previously.

Fixes #35
Signed-off-by: Antoni Segura Puimedon <532ff71c0f0c138e61afd0c77279be9f5bb6c4f0@midokura.com>

scharfie/wee-slack,rawdigits/wee-slack,wee-slack/wee-slack,trygveaa/wee-slack,megalithic/wee-slack