Fix #309 Fallback to no-emoji boot message when failing to load for some reason

[seratch]

This pull request resolves #309 . Doing this way eliminates the possibility of unicode loading errors when booting a Bolt app on any platforms.

Category (place an x in each of the [ ])

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements (place an x in each [ ])

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• 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.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

Merging #311 (27142d5) into main (ad622e4) will decrease coverage by 0.06%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #311      +/-   ##
==========================================
- Coverage   91.45%   91.38%   -0.07%     
==========================================
  Files         166      166              
  Lines        5278     5283       +5     
==========================================
+ Hits         4827     4828       +1     
- Misses        451      455       +4     

Impacted Files	Coverage Δ
slack_bolt/util/utils.py	84.61% <50.00%> (-9.51%)	⬇️

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