Improvements to use_targets(), tar_mermaid(), and error printing

targets 0.12.1

New features

• Add arguments legend and color to further configure tar_mermaid() (#848, @noamross).
• For HPC schedulers like SLURM and SGE, use_targets() now creates a job.sh script to run the pipeline as a cluster job (#839).

Enhancements

• Use lapply() to source scripts in use_targets(). Avoids defining a global variable for the file.
• Recursively find scripts to source in the use_targets() _targets.R file.
• Refactor error printing.