Cactus allows for negative error level

Currently, Cactus allows to set a negative error level through the command line option. This leads to CCTK_WARN(0,"") not necessarily aborting. While this is not wrong by itself, a lot of code assumes an abort at that point.

I propose to change CCTKi_SetErrorLevel() to only accept non-negative error levels and to change the command-parsing code to give an error message for that case as well. Patch is attached.

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