Catch integration key file not found in zuul

If the integration key file is not found this raises an uncaught IOError
that basically crashes zuul's main thread and prevents startup. This
exception is unfortunately never logged and so difficult to diagnose.

Change-Id: I78d35cce0ea8f4c4bf6662a38138bef245adafb9
Signed-off-by: Jamie Lennox <jamielennox@gmail.com>
diff --git a/zuul/driver/github/githubconnection.py b/zuul/driver/github/githubconnection.py
index 1d88aaa..2dbfd51 100644
--- a/zuul/driver/github/githubconnection.py
+++ b/zuul/driver/github/githubconnection.py
@@ -381,8 +381,12 @@
             self._github.login(token=api_token)
 
         if app_key_file:
-            with open(app_key_file, 'r') as f:
-                app_key = f.read()
+            try:
+                with open(app_key_file, 'r') as f:
+                    app_key = f.read()
+            except IOError:
+                m = "Failed to open app key file for reading: %s"
+                self.log.error(m, app_key_file)
 
         if (app_id or app_key) and \
                 not (app_id and app_key):