fix: shorebird engine should not crash if shorebird.yaml is missing

[eseidel]

PS C:\Users\micro\demo\hello_shorebird> shorebird run
Running app...
Flutter assets will be downloaded from https://download.shorebird.dev. Make sure you trust this source!

Using hardware rendering with device sdk gphone64 x86 64. If you notice graphics artifacts, consider enabling software rendering with "--enable-software-rendering".

Launching lib\main.dart on sdk gphone64 x86 64 in release mode...

Running Gradle task 'assembleRelease'...


Flutter assets will be downloaded from https://download.shorebird.dev. Make sure you trust this source!

Running Gradle task 'assembleRelease'...
   33.2s

√  Built build\app\outputs\flutter-apk\app-release.apk (9.0MB).

Installing build\app\outputs\flutter-apk\app-release.apk...     
 1,487ms



Flutter run key commands.

h List all available interactive commands.
c Clear the screen
q Quit (terminate the application on the device).

E/FlutterJNI( 5305): Failed to load shorebird.yaml

E/FlutterJNI( 5305): java.io.FileNotFoundException: flutter_assets/shorebird.yaml
E/FlutterJNI( 5305):    at android.content.res.AssetManager.nativeOpenAsset(Native Method)        
E/FlutterJNI( 5305):    at android.content.res.AssetManager.open(AssetManager.java:904)
E/FlutterJNI( 5305):    at android.content.res.AssetManager.open(AssetManager.java:881)
E/FlutterJNI( 5305):    at io.flutter.embedding.engine.FlutterJNI.init(Unknown Source:51)
E/FlutterJNI( 5305):    at m.d.e(Unknown Source:454)
E/FlutterJNI( 5305):    at io.flutter.embedding.engine.a.<init>(Unknown Source:184)
E/FlutterJNI( 5305):    at io.flutter.embedding.engine.a.<init>(Unknown Source:12)
E/FlutterJNI( 5305):    at io.flutter.embedding.android.e.G(Unknown Source:106)
E/FlutterJNI( 5305):    at io.flutter.embedding.android.e.p(Unknown Source:7)
E/FlutterJNI( 5305):    at io.flutter.embedding.android.d.onCreate(Unknown Source:13)
E/FlutterJNI( 5305):    at android.app.Activity.performCreate(Activity.java:8305)
E/FlutterJNI( 5305):    at android.app.Activity.performCreate(Activity.java:8284)
E/FlutterJNI( 5305):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1417)
E/FlutterJNI( 5305):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3626)
E/FlutterJNI( 5305):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3782)
E/FlutterJNI( 5305):    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:101)
E/FlutterJNI( 5305):    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
E/FlutterJNI( 5305):    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
E/FlutterJNI( 5305):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2307)   
E/FlutterJNI( 5305):    at android.os.Handler.dispatchMessage(Handler.java:106)
E/FlutterJNI( 5305):    at android.os.Looper.loopOnce(Looper.java:201)
E/FlutterJNI( 5305):    at android.os.Looper.loop(Looper.java:288)
E/FlutterJNI( 5305):    at android.app.ActivityThread.main(ActivityThread.java:7872)
E/FlutterJNI( 5305):    at java.lang.reflect.Method.invoke(Native Method)
E/FlutterJNI( 5305):    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
E/FlutterJNI( 5305):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:936)
D/flutter ( 5305): updater::logging: Logging initialized
E/flutter ( 5305): updater::c_api: Error initializing updater: Invalid Argument: yaml -> missing field `app_id`

E/flutter ( 5305): log_panics: thread '<unnamed>' panicked at 'Must call shorebird_init() before using the updater.': library/src/config.rs:68

E/flutter ( 5305):    0: <unknown>
E/flutter ( 5305):    1: <unknown>
E/flutter ( 5305):    2: <unknown>
E/flutter ( 5305):    3: <unknown>
E/flutter ( 5305):    4: <unknown>
E/flutter ( 5305):    5: <unknown>
E/flutter ( 5305):    6: <unknown>
E/flutter ( 5305):    7: <unknown>
E/flutter ( 5305):    8: <unknown>
E/flutter ( 5305):    9: <unknown>
E/flutter ( 5305):   10: art_quick_generic_jni_trampoline
E/flutter ( 5305):   11: nterp_helper
E/flutter ( 5305):   12: nterp_helper
E/flutter ( 5305):   13: nterp_helper
E/flutter ( 5305):   14: nterp_helper
E/flutter ( 5305):   15: nterp_helper
E/flutter ( 5305):   16: nterp_helper
E/flutter ( 5305):   17: nterp_helper
E/flutter ( 5305):   18: <unknown>

[bryanoltman]

To reproduce:

1. Remove shorebird.yaml from the app's pubspec.yaml's flutter.assets
2. Execute shorebird run