[cameras.pixelfly] Explicitly inform user about invalid settings

[natezb]

Currently, using e.g. an exposure time that's too long results in a pretty cryptic exception that comes directly from the SDK. We should catch this (or validate the input before using it) and say explicitly what setting is wrong.

This can be especially confusing b/c the valid exposure times depend on the shutter mode (single, double, video) and camera model.

Also, if single shutter mode is being used and the desired exposure is too long, we could let the user know that you can perform longer exposures in video mode.