Improve error message when installing plugins

[Giuspepe]

During installation of plugins we use sidekickDartRuntime to run pub get to download the dependencies of the plugin's installer. sidekickDartRuntime may depend on a different Dart SDK version than the actual sidekick CLI does. This may lead to an error like the following:

Process `/var/folders/c0/vw17q1tx6k50xkmzl4wx1pbh0000gn/T/iH2gEe/dashi sidekick plugins install flutterw_sidekick_plugin` exited with exitCode 255. Output:

    Installing flutterw_sidekick_plugin   for dashi
    Downloading from pub flutterw_sidekick_plugin...
    Running sidekickDartRuntime.dart with dart at /private/var/folders/c0/vw17q1tx6k50xkmzl4wx1pbh0000gn/T/iH2gEe/packages/dashi_sidekick/build/cache/dart-sdk/bin/dart
    Running sidekickDartRuntime.dart with dart at /private/var/folders/c0/vw17q1tx6k50xkmzl4wx1pbh0000gn/T/iH2gEe/packages/dashi_sidekick/build/cache/dart-sdk/bin/dart
    Installer downloaded
    Preparing flutterw_sidekick_plugin installer...
    Running sidekickDartRuntime.dart with dart at /private/var/folders/c0/vw17q1tx6k50xkmzl4wx1pbh0000gn/T/iH2gEe/packages/dashi_sidekick/build/cache/dart-sdk/bin/dart
    The current Dart SDK version is 2.14.0.
    
    Because dart_style >=2.2.0 <2.2.1 depends on analyzer ^2.0.0 and dart_style >=2.2.1 <2.2.2 depends on analyzer >=2.6.0 <4.0.0, dart_style >=2.2.0 <2.2.2 requires analyzer >=2.0.0 <4.0.0.
    And because dart_style >=2.2.2 <2.2.3 depends on analyzer ^3.3.1, dart_style >=2.2.0 <2.2.3 requires analyzer >=2.0.0 <4.0.0.
    And because dart_style >=2.2.3 <2.2.4 depends on analyzer >=3.3.1 <5.0.0 and dart_style >=2.2.4 requires SDK version >=2.17.0 <3.0.0, dart_style >=2.2.0 requires analyzer >=2.0.0 <5.0.0.
    So, because flutterw_sidekick_plugin depends on both analyzer ^5.0.0 and dart_style ^2.2.0, version solving failed.
[e] Unhandled exception:
[e] /private/var/folders/c0/vw17q1tx6k50xkmzl4wx1pbh0000gn/T/iH2gEe/packages/dashi_sidekick/build/cache/dart-sdk/bin/dart pub get 
[e] exit: 1

The sidekick CLI is using Dart SDK version 2.18, so it should be able to use the plugin. However, because its sidekickDartRuntime is on Dart SDK version 2.14, pub get fails.

A clearer error message should be printed in this case. The solution would be to update the sidekickDartRuntime's Dart SDK version (see #149)

[passsy]

Idea: Add a sidekick upgrade-embedded-sdk command, we can print to overcome such issues. Exactly what you wrote in #149