Description

Catch and display version check issues instead of crashing.

New text:

Doctor summary (to see all details, run flutter doctor -v):
[!] Flutter (Channel unknown, v0.0.0-unknown, Mac OS X 10.14.6 18G87, locale en-US)
    ✗ Command exited with code 69: git -c log.showSignature=false log -n 1 --pretty=format:%ad --date=iso
      Standard error: 
      Agreeing to the Xcode/iOS license requires admin privileges, please run “sudo xcodebuild -license” and then retry this command.

Related Issues

Fixes #28939

Tests

Added new doctor_test.

Checklist

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• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
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Breaking Change

• Yes, this is a breaking change (Please read [Handling breaking changes]). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.