Telemetry: Simplify exception handling of failed websocket connections.

Previously, an intermediary exception InspectorException was generated. This
was caught and then converted into a more common Telemetry exception. Now the
connection failure throws a standard Telemetry exception directly.

BUG=460625

Review URL: https://codereview.chromium.org/955263003

Cr-Commit-Position: refs/heads/master@{#319161}

tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py

@@ -74,10 +74,12 @@ class TabListBackend(inspector_backend_list.InspectorBackendList):
   def CreateWrapper(self, inspector_backend):
     return tab.Tab(inspector_backend, self, self._browser_backend.browser)
 
-  def _HandleDevToolsConnectionError(self, err_msg):
+  def _HandleDevToolsConnectionError(self, error):
     if not self._browser_backend.IsAppRunning():
-      raise exceptions.BrowserGoneException(self.app, err_msg)
+      error.AddDebuggingMessage('The browser is not running. It probably '
+                                'crashed.')
     elif not self._browser_backend.HasBrowserFinishedLaunching():
-      raise exceptions.BrowserConnectionGoneException(self.app, err_msg)
+      error.AddDebuggingMessage('The browser exists but cannot be reached.')
     else:
-      raise exceptions.DevtoolsTargetCrashException(self.app, err_msg)
+      error.AddDebuggingMessage('The browser exists and can be reached. '
+                                'The devtools target probably crashed.')

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

@@ -22,10 +22,6 @@ from telemetry.timeline import model as timeline_model_module
 from telemetry.timeline import trace_data as trace_data_module
 
 
-class InspectorException(Exception):
-  pass
-
-
 class InspectorBackend(object):
   def __init__(self, app, devtools_client, context, timeout=60):
     self._websocket = inspector_websocket.InspectorWebsocket()
@@ -44,7 +40,7 @@ class InspectorBackend(object):
     try:
       self._websocket.Connect(self.debugger_url)
     except (websocket.WebSocketException, exceptions.TimeoutException) as e:
-      raise InspectorException(e.msg)
+      self._HandleError(e)
 
     self._console = inspector_console.InspectorConsole(self._websocket)
     self._memory = inspector_memory.InspectorMemory(self._websocket)

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

@@ -77,9 +77,9 @@ class InspectorBackendList(collections.Sequence):
       try:
         backend = self._devtools_context_map_backend.GetInspectorBackend(
             context_id)
-      except inspector_backend.InspectorException:
-        err_msg = sys.exc_info()[1]
-        self._HandleDevToolsConnectionError(err_msg)
+      except exceptions.Error as e:
+        self._HandleDevToolsConnectionError(e)
+        raise e
       # Propagate KeyError from GetInspectorBackend call.
 
       wrapper = self.CreateWrapper(backend)
@@ -114,10 +114,10 @@ class InspectorBackendList(collections.Sequence):
       if context_id not in self._filtered_context_ids:
         del self._wrapper_dict[context_id]
 
-  def _HandleDevToolsConnectionError(self, err_msg):
-    """Call when handling errors in connecting to the DevTools websocket.
+  def _HandleDevToolsConnectionError(self, error):
+    """Called when handling errors in connecting to the DevTools websocket.
 
-    This can be overwritten by sub-classes to further specify the exceptions
-    which should be thrown.
+    This can be overwritten by sub-classes to add more debugging information to
+    errors.
     """
-    raise exceptions.DevtoolsTargetCrashException(self.app, err_msg)
+    pass