fix(shorebird_cli): shorebird init fail on flavor detection error

[felangel]

Description

• fix(shorebird_cli): shorebird init fail on flavor detection error
  • detect missing gradlew and provide instructions for how to resolve.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (cf312f5) 100.00% compared to head (ae6adba) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #735   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           72        72           
  Lines         2539      2546    +7     
=========================================
+ Hits          2539      2546    +7     

Impacted Files	Coverage Δ
...s/shorebird_cli/lib/src/commands/init_command.dart	100.00% <100.00%> (ø)
.../shorebird_cli/lib/src/shorebird_flavor_mixin.dart	100.00% <100.00%> (ø)

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