Running "mcexec dfsafds" did not print any message in normal use.
Rather than looking for which message shows in debug and turn in into
eprintf, add a single coherent message (more shell-like) at the end and
turn other messages off.
There is a small loss of information but this is equivalent to what
shells give (a single errno value with no details), and it is now easy
to add --debug to mcexec to see more information if required
Change-Id: Id2c3a47880b7d1d7467883351e6e7af561f91bbf
a59c55c188