[Telemetry] Make the logging message when screenshot capturing fail less verbose.

Adding the stack trace of screenshot failure will make it's harder for perf
sherrifs to diagnore issue. Switch this to log the exception object instead.

BUG=549326

Review URL: https://codereview.chromium.org/1408063008

Cr-Commit-Position: refs/heads/master@{#357012}

tools/telemetry/telemetry/page/shared_page_state.py

@@ -136,9 +136,9 @@ class SharedPageState(story.SharedState):
         logging.warning(
             'Either tab has crashed or browser does not support taking tab '
             'screenshot. Skip taking screenshot on failure.')
-    except Exception:
-      exception_formatter.PrintFormattedException(
-        msg='Error when trying to capture screenshot:')
+    except Exception as e:
+      logging.warning('Exception when trying to capture screenshot: %s',
+                      repr(e))
 
   def DidRunStory(self, results):
     if self._finder_options.profiler: