ae412a7 routed filesystem failures to exit 2, but the analyzer throws its own
FileSystemException (package:analyzer/file_system), which the dart:io catch
in bin/pipe.dart never matched. A project whose package_config still named a
moved Flutter SDK therefore crashed with exit 255 and a stack trace. Found
by dogfooding on word_alive after its SDK moved.
Analyzer.create now turns that into a PipeException naming the SDK path, with
the remedy to re-run flutter pub get or pass --sdk.
