vcl: don't throw misleading exceptions on every failed access

During the build different instances of gengal will create a temp file
called "instdir/share/config/cdefghij.klm", and enumerate the
directories in instdir/share/config; it may happen that the enumeration
sees the temp file, and it is removed before fetchFileStatus is called,
resulting in E_NOENT rc and the exception.

The FileIsValidIconTheme() should be able to handle most errors here.

(regression from 60954a8a96a66ff11e06b850838f3d16e8e1625f)

Change-Id: Id9361a37b8569d01509efcc8cda2bc17e9eabcd8