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

**Description**

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

**Steps To Reproduce**

1. Run a Mason Generator:
```dart
final generator = await MasonGenerator.fromBundle(myBundle);
await generator.hooks.preGen(vars: {'name':'Dash'});
```

2. See error:
```txt
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](https://github.com/felangel/mason/blob/8bf695d1dee6ca3dc1a44241dcec00ee3603653d/packages/mason/lib/src/hooks.dart#L215).

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`](https://github.com/felangel/mason/blob/8bf695d1dee6ca3dc1a44241dcec00ee3603653d/packages/mason/lib/src/hooks.dart#L161) calls [`_runHook`](https://github.com/felangel/mason/blob/8bf695d1dee6ca3dc1a44241dcec00ee3603653d/packages/mason/lib/src/hooks.dart#L275), that calls [`_installDependencies`](https://github.com/felangel/mason/blob/8bf695d1dee6ca3dc1a44241dcec00ee3603653d/packages/mason/lib/src/hooks.dart#L229) that calls [`_dartPubGet`](https://github.com/felangel/mason/blob/8bf695d1dee6ca3dc1a44241dcec00ee3603653d/packages/mason/lib/src/hooks.dart#L214) that calls `dart pub get`.