CLI exits on first command when a credential is invalid.

Currently CLI goes back to a normal command line loop showing a
prompt even when a previous command failed because of an invalid
credential. This is confusing as it can appear as if authentication
was successful and CLI was ready to take commands. The new behavior
is that CLI will exit upon a first command if a credential is
invalid, displaying a corresponding error message.

Fix MN-718

Change-Id: I2ceb940f35c7418edee3601a7fc55874e24436dd