Ignore errors while trying to read the uidvalidity file. If there is

Ignore errors while trying to read the uidvalidity file.  If there is
a problem reading the file, this means that we will delete the cache,
rather than signal an error to the user.  This may be the wrong
behaviour in some obscure cases -- e.g., perhaps a networked file
system where the network fails after checking that the cache directory
is in fact a directory --, but this is useful for cases where the
cache directory was just set up wrong, perhaps because Scheme was
interrupted.