More warnings on mistakes

[dehnert]

People mis-fill things out with significant frequency, and we should provide some sort of warning. Two classes that have come up:

• Checking somebody in twice (I think it's possible this is "if somebody gets checked in, and then pays, we create an attendance record both times" -- that doesn't necessarily warrant any sort of warning, just silently ignoring or adopting the attendance record) (Note: Handle double-submitted attendance better #14 addresses the specific "check-in, then pay" case.)
• Choosing the wrong week (I think I did this today -- possibly I hit "back" to get the list of dances and clicked the blue one, without refreshing to update the blue highlight) -- I think probably we want a warning if doing gate for a dance more than, say, 3 hours before it starts or 6 hours after.

I'm not sure if we want a hard error, or give a soft error and have some mechanism for ignoring them.

[dehnert]

The latter came up again today (viz https://discord.com/channels/843577671085850644/948444185000804403/1326355452530917428)

Another issue is that whenever we give a hard error, we should probably store that (client or server) so that if somebody doesn't notice the (async) error we can see "oh they tried to check in Adam Smith but forgot the payment method, it was probably cash so go add that". This part may tie in with #37 in that better JS hopefully comes with client side ~logging of actions.