Catch exception raised when Xephyr can't be found

If Xephyr isn't installed currently src/gnome-shell raises
an exception which isn't at all obvious without looking at
the source. This commit catches the exception and replaces
it with a more informative error message.

src/gnome-shell.in

@@ -11,6 +11,7 @@ import subprocess
 import sys
 import tempfile
 import time
+import errno
 
 def find_cmd (cmd_list):
     """
@@ -70,10 +71,17 @@ def start_xephyr():
         raise RuntimeError("xauth failed")
 
     # Launch Xephyr
+    try:
         xephyr = subprocess.Popen(["Xephyr", display,
                                    "-auth", xauth_file,
                                    "-screen", options.geometry,
                                    "-host-cursor"])
+    except OSError, e:
+        if e.errno == errno.ENOENT:
+            print "Could not find Xephyr."
+            sys.exit(1)
+        else:
+            raise
     os.environ['DISPLAY'] = display
     os.environ['XAUTHORITY'] = xauth_file