[Telemetry] Include debugging info when foreground tab is missing

telemetry/telemetry/core/exceptions.py

Index: telemetry/telemetry/core/exceptions.py
diff --git a/telemetry/telemetry/core/exceptions.py b/telemetry/telemetry/core/exceptions.py
index 743b497d0791b950b76481a8d0029796aa54d22e..b746bce4d5f3eef640a1e2d3d81ff9d687124ff6 100644
--- a/telemetry/telemetry/core/exceptions.py
+++ b/telemetry/telemetry/core/exceptions.py
@@ -132,6 +132,10 @@ class BrowserConnectionGoneException(BrowserGoneException):
     super(BrowserConnectionGoneException, self).__init__(app, msg)
 
 
+class TabMissingError(Error):
+  """Represents an error when an expected browser tab is not found."""
+
+
 class ProcessGoneException(Error):
   """Represents a process that no longer exists for an unknown reason."""