Fixes #133585.

This PR elects to add a new catch within _handleToolError that checks any uncaught error. This new catch will exit the tool without crashing provided the following conditions are met:

1. the error is a ProcessException,
2. the error message contains git somewhere in the message (we don't match on the entire string in case it changes or is locale-dependent), and
3. git does not appear to be runnable on the host system (ProcessManager.canRun returns false for git).

This is preferable to checking for runnability of git before we run it for 1) its simplicity and 2) lack of performance penalty for users that already have git installed (almost every single one).

This PR also does some light refactoring to runner_test.dart to make room for tests that aren't related to crash reporting.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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