pygrub: verify chosen kernel really exists

Verify that the chosen kernel really exists, and fail with an informative error
if it does not.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

diff -r 32034d1914a6 tools/pygrub/src/pygrub
--- a/tools/pygrub/src/pygrub
+++ b/tools/pygrub/src/pygrub
@@ -821,10 +821,15 @@ if __name__ == "__main__":
     if not fs:
         raise RuntimeError, "Unable to find partition containing kernel"
 
+    # Does the chosen kernel really exist ?
+    try:
+        data = fs.open_file(chosencfg["kernel"]).read()
+    except:
+        raise RuntimeError, "The chosen kernel does not exist"
+
     if not_really:
         bootcfg["kernel"] = "<kernel:%s>" % chosencfg["kernel"]
     else:
-        data = fs.open_file(chosencfg["kernel"]).read()
         (tfd, bootcfg["kernel"]) = tempfile.mkstemp(prefix="boot_kernel.",
                                                     dir=output_directory)
         os.write(tfd, data)