Don't hide errors in PlatformFreeBSD::ResolveExecutable

If error is already set then there's no reason to replace it with a
generic "<file> does not exist" message.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@189686 91177308-0d34-0410-b5e6-96231b3b80d8