Explicitly check spreadsheet size and error on too-large sheets

[jpolitz]

This is a general resource issue, but comes up most commonly w/spreadsheets. There's always going to be some limit on what can be loaded based on the machine you're running on. However, reaching the system limits generally gives bad browser-crashy behavior. We should explicitly limit the size of spreadsheets in the default loader to something like 10k rows (or maybe 100k total cells).

We could provide another function that will skip the error check for brave souls with beefy machines or who want to test the limits, but the default should give quick, non-crashy feedback.

Thanks Eric Allatta for reporting.