test-save-errors: add tests for inaccessible metadata

test-save-errors: add tests for inaccessible metadata

Add test cases for when metadata for a file and directory cannot
be read (causing an IOError instead).

Note that this test fails if the previous two patches aren't
applied.

Note also that if such an error ever happened without the patch
to save, the repository is essentially corrupt and there no way
to figure out _which_ file didn't get metadata.

Note that this uses some python trickery to force an IOError in
metadata reading.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
[rlb@defaultvalue.org: name test in commit summary line]
Tested-by: Rob Browning <rlb@defaultvalue.org>