validation text: state errors making validation fails

[jcolomb]

errors (in red in the current validation text) make the spreadsheet not usable (the show infosheet button fails).

It should be clearer for the user that it is the case and that the infosheet cannot be used.

Alternatively, we could have no "error validation" but only warnings.

[jcolomb]

we go for a message stating that the infosheet cannot be used.
we should still be able to see the spreadsheet, with a warning, maybe even the problematic cells showed in red (via javascript)