[Tool] Gracefully handle WDAC/AppLocker blocks for impellerc

[bkonyi]

On secured Windows environments (using WDAC or AppLocker), the execution of impellerc.exe is blocked by the OS, throwing a fatal ProcessException and crashing the build with a raw stack trace.

This change implements:

1. Detection for WDAC/AppLocker block exceptions (checking for error code 1260 and policy-related error text).

2. A highly visible, user-friendly boxed error message advising the user to contact their administrator to whitelist the binary.

3. Respects the fatal flag: throws a clean ToolExit during builds, but returns false gracefully during interactive hot reload sessions to allow the session to continue.

Fixes #188447

[fluttergithubbot]

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[gemini-code-assist]

Code Review

This pull request adds handling for when the Impeller shader compiler is blocked by system security policies, logging a friendly error message and throwing a ToolExit. It also suppresses stack traces for ToolExit exceptions in BundleBuilder and WebBuilder. Feedback suggests tracking whether the security block error has already been logged to prevent flooding the console with duplicate warning messages during non-fatal runs.

[chingjun]

LGTM with nits