Fix echo'ing of exit status or exception on exit

cli.py main() had a "try/except Exception, e" added
to it. This exception handlers was not needed except
that code was raising Exceptions() as control flow,
instead of a more specific exception for the case
of trying to write to an already existing file. So
manifest_commands.py was changed to handle
EnvironmentError (base class of IOError/OSError)
itself, and the try/except Exception was removed.

That handler would call system_exit(exit_status,
exception) but system_exit expects (exit_status, msg),
and eventually call sys.exit(). On RHEL5, this was
causing the exit status to get echoed to stderr.
On RHEL5 python2.4, systemExit exception is
a subclass of Exception, so it was getting
handled by systemExit as well, resulting in
the echo'ing to stderr. On python 2.5+, it
is based on BaseException.

Error handling for dump-manifest was improved
to show a specific error for existing file,
and a generic error for other errors.
Merge pull request #608 from candlepin/alikins/fix_cli_exception

Fix echo'ing of exit status or exception on exit