[ddc] Unexpected errors thrown from the hot reload test suite are not always plumbed to the console output

[nshahan]

Some example output:

hot_reload_test(class_field_change_type): Executing VM test.
hot_reload_test(class_field_change_type): FAILED with:
  Reloading: file:///var/folders/pp/5_07cs6d0mz5tsbbj1cx2vlh009rgp/T/ZDiIQ9/class_field_change_type/generation1/class_field_change_type.dill
Reloading: file:///var/folders/pp/5_07cs6d0mz5tsbbj1cx2vlh009rgp/T/ZDiIQ9/class_field_change_type/generation2/class_field_change_type.dill
Unhandled exception:
reloadSources: (-32000) Service connection disposed
#0      VmService._call (package:vm_service/src/vm_service.dart:1912:7)
#1      VmService.reloadSources (package:vm_service/src/vm_service.dart:1553:7)
#2      HotReloadHelper.reloadNextGeneration (package:reload_test/src/_vm_reload_utils.dart:96:41)
#3      hotReload (package:reload_test/src/_vm_reload_utils.dart:25:27)
#4      main (hot-reload-test:///main.dart:21:9)
<asynchronous suspension>

The actual message from the unhandled exception in this case should be type 'String' is not a subtype of type 'int' of 'function result'.

The message and the stack trace should be added to the failure output.