The `replace_version_and_checksum` method handles a `CaskInvalidError`
when loading a cask (handling casks that aren't valid on Linux) but
we can sometimes still encounter an error when bumping a cask with
on_system blocks. For example, bumping `displaylink` will produce a
`Cask 'displaylink' is unreadable: undefined method 'csv' for nil`
error when `SimulateSystem` runs as Linux, as the cask interpolates
`version.csv.first` in a `license` string but `version` isn't set on
Linux.
This adds `Cask::CaskUnreadableError` to the `rescue` arguments,
which accounts for this particular situation (allowing `displaylink`
to be bumped like before).
612fec47f3