Populate __context__ with retcode; Fixes issue #5518

Returns the proper retcode when running something like "salt-call cmd.run lsdfds --return local":

`{'fun': 'cmd.run', 'jid': '20130727172909251080', 'return': '/bin/bash: lsdfds: command not found', 'retcode': 127, 'id': 'Helios'}`

Previously `__context__` did not have `retcode` defined, and defaulted to 0 (in line 70 of salt/cli/caller).