cache plugin fails when previously saved file is corrupted

[eshedg]

problem

in some cases, the local file from a previous 'save-cache' step may be corrupted (with EOFs etc).

desired behavior

the plugin should be able to ignore the local file and revert to default behavior (i.e. download the file and extract it).
the default behavior will also take care of overwriting the corrupted local file for the next time.