copy_file() always ignore missing file

[gsmolk]

It is ok for backup, but in case of restore with --no-validate flag it could lead to restoring backup with missing non-data files. So I think missing_ok flag must be added to copy_file()

[gsmolk]

Test:

======================================================================
FAIL: test_lost_non_data_file (tests.restore.RestoreTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/restore.py", line 2335, in test_lost_non_data_file
    self.output, self.cmd))
AssertionError: Expecting Error because of non-data file dissapearance.
 Output: WARNING: Backup PRYGXB is used without validation.
INFO: Restore of backup PRYGXB completed.

[gsmolk]

backup_data_file() has the same problem, which can potentially lead to file loss in merge, although it is highly unlikely, but better safe than sorry

[gsmolk]

PR: #75