Inform admin when spreadsheet is too large to cache #12
Description
Currently the spreadsheet data is zipped and stored in Memcached when accept_staleness is set to a non-zero value.
Memcached has a size limitation of 1Mb and if setting fails because of the compressed spreadsheet being larger than this, an error is logged in the request log, but no notification is sent to user nor the admin.
It would be a good idea to add for example an email which would be sent to the admin (max 1 per day per spreadsheet) when someone tries to enable caching on a spreadsheet which does not fit into memory in compressed form.