Return a bad compiler result instead of rethrowing in ddk.

This matches the DDC behavior and fixes an issue with duplicate stack traces being logged.

Change-Id: I3e968c3ca36875ccc6d92bf77a357af83ea8d6cb
Reviewed-on: https://dart-review.googlesource.com/c/86940
Reviewed-by: Jenny Messerly <jmesserly@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>

pkg/dev_compiler/lib/src/kernel/command.dart

@@ -49,7 +49,7 @@ any other information that may help us track it down. Thanks!
 $error
 $stackTrace
 ''');
-    rethrow;
+    return CompilerResult(70);
   }
 }