ltldo: improve error messages
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.