Update and clean up build-macOS.sh

[ex-nerd]

• Fix pyenv setup so it's used more consistently (and won't be interfered with by users who use different tools for managing python envs)
• Switch to using echo -e so enhanced/hidden character sequences are honored
• Consolidate error handling into its own function
• Add error checking and messages after most commands so the script exits with helpful information when there is a problem
• Clean up a few whitespace issues
• Switch to using double-bracket bash conditionals instead of the single-bracket alias for the test program

[stephenhouser]

Nice and thank you!