Stop failed m_mktemp2() that give directory reporting get_temp_dir().

Stop failed m_mktemp2() that give directory reporting get_temp_dir().

The first parameter of m_mktemp2(), if not NULL, is a path and the
basename is used as the directory to hold the new temporary file, but
the error message on failing to create that used get_temp_dir() and
that's often somewhere else, e.g. `mhbuild /dev/null' attempts /dev/...
Don't attempt to state the failed path in the error message as at least
that's not misleading.  Perhaps the function needs some other indication
of success so it can pass back the attempted filename that failed.