feat(shorebird_cli): improve "shorebird not initialized" error messages

[bryanoltman]

Description

When a user is running a shorebird command outside of a shorebird directory, or if a user's pubspec.yaml is missing shorebird.yaml as a flutter asset, we print the error message Shorebird is not initialized. Did you run "shorebird init"?, which is confusing to users (see https://discord.com/channels/1030243211995791380/1215306109137584128).

This change updates the error message to help the user pinpoint the problem more quickly. If no shorebird.yaml is found, it now tells them:

Unable to find shorebird.yaml. Are you in a shorebird app directory?
If you have not yet initialized your app, run shorebird init to get started.

If the pubspec is missing shorebird.yaml as a flutter asset, it tells them:

Your pubspec.yaml does not have shorebird.yaml as a flutter asset.
To fix, update your pubspec.yaml to include the following:

  flutter:
    assets:
      - shorebird.yaml # Add this line

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

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (c0a2323) to head (c73ec8f).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1779   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          167       167           
  Lines         5191      5197    +6     
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+ Hits          5191      5197    +6     

Flag	Coverage Δ
shorebird_cli	100.00% <100.00%> (ø)

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[eseidel]

And this is why we hang out with our customers on discord :)