Adds friendlier config error messages to Greet and React example app

[aaronschachter]

Summary

Outputs a readable error message instead of a stack trace per #818. Also adds a few minor cleanup tweaks:

• Fixes a typo in README
• Adds .env, storage dir, and EOL to .gitignore
• Adds link to ngrok in README

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[aoberoi]

small nit: could we put this helper function at the bottom of the file (or even import it from another file)? i think having this so close to the top captures too much attention for a beginner who is coming here to learn about how to use the @slack/events-api package.

[codecov]

Codecov Report

Merging #841 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #841   +/-   ##
=======================================
  Coverage   95.73%   95.73%           
=======================================
  Files          12       12           
  Lines         680      680           
  Branches      145      145           
=======================================
  Hits          651      651           
  Misses         11       11           
  Partials       18       18

Flag	Coverage Δ
#eventsapi	89.61% <ø> (ø)	⬆️
#interactivemessages	96.62% <ø> (ø)	⬆️
#webapi	98.5% <ø> (ø)	⬆️
#webhook	87.5% <ø> (ø)	⬆️

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[seratch]

@aaronschachter Thanks a lot for your contribution and we are sorry for our extremely slow response here. Your revised changes look great to me. Let me merge this PR now.