ENH: fail gracefully if buffer exceeds total available buffer size

[RebeccaWPerry]

When the buffer is full, the software simply freezes. This happens at frame 60355 when using 1024x1024 8-bit tifs on the uberscope computer. It would be more helpful if a warning popped up to suggest that the user change the buffer size on the Camera tab. When you change this to a smaller buffer, the live feed continues gracefully-- so that's good.