Telemetry: Keep original exception's stacktrace in inspector_backend.

InspectorBackend converts exceptions generated by the websocket library into
Telemetry exceptions. In the process, it was losing the original stacktrace.

BUG=NONE

Review URL: https://codereview.chromium.org/990153003

Cr-Commit-Position: refs/heads/master@{#319821}

tools/telemetry/telemetry/core/backends/chrome_inspector/inspector_backend.py

@@ -235,7 +235,7 @@ class InspectorBackend(object):
     new_error.AddDebuggingMessage(original_error_msg)
     self._AddDebuggingInformation(new_error)
 
-    raise new_error
+    raise new_error, None, sys.exc_info()[2]
 
   def _AddDebuggingInformation(self, error):
     """Adds debugging information to error.