Improve message for missing or invalid APP_ENV

[samdark]

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Fixed issues	-

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.22%. Comparing base (97fbe8b) to head (72440f3).

Files with missing lines	Patch %	Lines
src/Environment.php	16.66%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #399      +/-   ##
============================================
- Coverage     78.08%   77.22%   -0.87%     
- Complexity       28       29       +1     
============================================
  Files            11       11              
  Lines           178      180       +2     
============================================
  Hits            139      139              
- Misses           39       41       +2     

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[vjik]

Add line to changelog