[Feature] Make benchmark tests more robust

[stweil]

Currently workflows/execute_workflows.sh sets shell option errexit (shell command set -euo pipefail).
Therefore the script terminates if any OCR-D processor exits with a runtime error.

Test code should be robust enough to handle errors in the tested programs and continue with additional test cases.

Maybe setting the errexit option could also be done conditionally, depending on a user parameter for the script.