Fix 'non-existing get' in InMemoryStorage

Returning a 'failed promise' in case of
'not found' instead of throwing an
exception, to be consistent with the
ZoomObjectMapper behavior.

Change-Id: I32dfa22615b1f1101065b6d4363ae54be8d8c759
Signed-off-by: Ernest Artiaga <ernest@midokura.com>
Merge "Fix 'non-existing get' in InMemoryStorage"