fix: capture ProcessException when hook generation fails due to dart pub get

[alestiago]

Description

Running hooks using the Mason API throws a ProcessException if Dart executable is not installed.

Steps To Reproduce

1. Run a Mason Generator:

final generator = await MasonGenerator.fromBundle(myBundle);
await generator.hooks.preGen(vars: {'name':'Dash'});

2. See error:

ProcessException: No such file or directory
  Command: /bin/sh -c 'dart' 'pub' 'get'

Expected Behavior

I would expect the error to be captured, it is currently not being captured.

I would expect a better error message that outlined the issue comes from Mason itself (since it is running the dart pub get internally) with a verbose description on how to resolve.

Additional Context

• The dart pub get is coming from Mason, see that preGen calls _runHook, that calls _installDependencies that calls _dartPubGet that calls dart pub get.