Better error messages

[DenuxPlays]

If I have an error in a make_config.yaml file the error is not usable.
Example:

Successfully built Directory: 'build/linux/x64/release/bundle' in 55s
type 'Null' is not a subtype of type 'String' in type cast
#0      new MakeAppImageConfig.fromJson (package:flutter_app_packager/src/makers/appimage/make_appimage_config.dart:48:40)
#1      MakeAppImageConfigLoader.load (package:flutter_app_packager/src/makers/appimage/make_appimage_config.dart:136:31)
#2      FlutterAppPackager.package (package:flutter_app_packager/src/flutter_app_packager.dart:40:39)
#3      UnifiedDistributor.package (package:unified_distributor/src/unified_distributor.dart:197:51)
<asynchronous suspension>
#4      CommandRunner.runCommand (package:args/command_runner.dart:212:13)
<asynchronous suspension>
#5      main (file:///home/runner/.pub-cache/hosted/pub.dev/fastforge-0.6.6/bin/main.dart:10:10)
<asynchronous suspension>
Unhandled exception:
type 'Null' is not a subtype of type 'String' in type cast
#0      new MakeAppImageConfig.fromJson (package:flutter_app_packager/src/makers/appimage/make_appimage_config.dart:48:40)
#1      MakeAppImageConfigLoader.load (package:flutter_app_packager/src/makers/appimage/make_appimage_config.dart:136:31)
#2      FlutterAppPackager.package (package:flutter_app_packager/src/flutter_app_packager.dart:40:39)
#3      UnifiedDistributor.package (package:unified_distributor/src/unified_distributor.dart:197:51)
<asynchronous suspension>
#4      CommandRunner.runCommand (package:args/command_runner.dart:212:13)
<asynchronous suspension>
#5      main (file:///home/runner/.pub-cache/hosted/pub.dev/fastforge-0.6.6/bin/main.dart:10:10)
<asynchronous suspension>

it would be nice to state what key should not be null.