Use ltldo:... instead of error:... and warning:... and also improve
the diagnostic displayed after a translation failure to mention the
tool and formula.

Incidentally, this fixes a spurious test case failure observed by
Philipp Schlehuber on CentOS7.7 where glibc 2.17 is installed.  With
this system, when posix_spawn() starts a binary that does not exist,
it returns success and let the child die with exit code 127.  On more
recent glibc, posix_spawn() manages to return execve()'s errno, as if
the child had not been created.  We handle those two different ways to
fail, but before this patch one used to print "error:..." and the
other "ltldo:...".

* bin/ltldo.cc: Display the program_name in error message.  Display
the command name and formula on translation failure.
* tests/core/ltldo.test: Adjust test case.
* NEWS: Mention the fix.