Added unit test for stack traces in telemetry crashes.

tools/telemetry/telemetry/core/exceptions.py

Index: tools/telemetry/telemetry/core/exceptions.py
diff --git a/tools/telemetry/telemetry/core/exceptions.py b/tools/telemetry/telemetry/core/exceptions.py
index 561376bf30fb9b862bcb31f622a4d4beb4fab33c..4ece41cb1a41a0d5135fec792c29b91428e696b6 100644
--- a/tools/telemetry/telemetry/core/exceptions.py
+++ b/tools/telemetry/telemetry/core/exceptions.py
@@ -66,6 +66,10 @@ class AppCrashException(Error):
       except Exception as err:
         logging.error('Problem when trying to gather standard output: %s' % err)
 
+  @property
+  def stack_trace(self):
+    return self._stack_trace
+
   def __str__(self):
     divider = '*' * 80
     debug_messages = []