devtool: reset: avoid errors in case file no longer exists

devtool: reset: avoid errors in case file no longer exists

If you manually delete files in the workspace layer (which you really
shouldn't) it was possible to get yourself into the situation where you
couldn't reset because we were attempting to check if the file had been
modified and erroring out if it couldn't be opened. If the file's not
there anymore there's not much point checking if it needs to be
preserved, just skip it.

(From OE-Core rev: d2d352f7b747a97a54df9d20eb1455d166aa1ee3)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>