Improve logging when input files are bad.

In the following log, the error should be the last printed message.

Reproduce

robin batsim_nosched_badinput.yaml

Log

+ batsim -p /home/carni/proj/batsim/platforms/nosuchplatform.xml -w /home/carni/proj/batsim/workload_profiles/test_workload_profile.json -e /tmp/robin/batsim_nosched_badinput/out --batexec
[0.000000] /home/carni/proj/batsim/src/batsim.cpp:271: [batsim/ERROR] Platform file '/home/carni/proj/batsim/platforms/nosuchplatform.xml' cannot be read.
[0.000000] [batsim/INFO] Workload '44f067' corresponds to workload file '/home/carni/proj/batsim/workload_profiles/test_workload_profile.json'.

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