Fix overly specific detection of non-UTF8 files in analyzer bot. (#56295)

An upcoming Dart SDK change
(https://github.com/dart-lang/sdk/commit/fa2fd41166db35afa4777e63f900e83d25709c5c)
changes the precise text of the exception generated by `utf8.decode`
if a non-UTF8 file is found.  This is causing a breakage in the Dart
team's `flutter-analyze` bot (and will presumably cause a breakage in
the corresponding Flutter bot as soon as this change is rolled into
Flutter).  To avoid this breakage, the bot shouldn't rely on the exact
exception text; it is sufficient to simply catch a FormatException.