camera zoom of 0 causes hang

[cyrush]

Accidentally had camera zoom set to 0 (which doesn't make much sense) and noticed that my render hung.
We should provide an error, not hang.

[cyrush]

Here is why zero is an issue:

ascent/src/libs/ascent/runtimes/flow_filters/ascent_runtime_conduit_to_vtkm_parsing.cpp

Line 68 in ef12b5d

double vtkm_zoom = log(in_zoom) / log(4.0);

log(0) is bad news.