Handle fletcher32 errors for online formats

[paulmueller]

...because normally data online is not currupt. dclab should attempt to reopen the file in this case.

[paulmueller]

This could also explain error messages shown in e.g. #244

[paulmueller]

This should happen extremely rarely. I believe the majority of the issues is a result of presigned S3 URLs expiring. It should not happen regularly with dclab 0.64.

Please reply here if you are seeing checksum errors with remote data.