[Telemetry] Include debugging info when foreground tab is missing

telemetry/telemetry/internal/browser/browser.py

Index: telemetry/telemetry/internal/browser/browser.py
diff --git a/telemetry/telemetry/internal/browser/browser.py b/telemetry/telemetry/internal/browser/browser.py
index ff6a689ce500cea5b36888b2c337d9af0c9d555a..4b4a244a98a325a8e4591612e458688dfe9d2379 100644
--- a/telemetry/telemetry/internal/browser/browser.py
+++ b/telemetry/telemetry/internal/browser/browser.py
@@ -103,7 +103,7 @@ class Browser(app.App):
       # tab is always tab 0, which will be the first one that isn't hidden
       if self._tabs[i].EvaluateJavaScript('!document.hidden'):
         return self._tabs[i]
-    raise Exception("No foreground tab found")
+    raise exceptions.TabMissingError("No foreground tab found")
 
   @property
   @decorators.Cache