Do not assume file must exist after an open call

[wangvsa]

Fix for issue #776 :

unifyfs_fid_open function assumes the file must exist after the open call, it throws an error if it's not.
This is not true because a file may be opened without O_CREAT.
Fix: changed LOGERR to LOGDBG and modified some related comments.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Testing (addition of new tests or update to current tests)
• Documentation (a change to man pages or other documentation)

[adammoody]

Thanks, @wangvsa