ENH: PyDarshan job summary print warning and bail on logs with no data

[shanedsnyder]

Detect empty logs, embed a warning in the report, and bail on typical report generation.

Fixes #845.

I won't close #734, as that issue is more tied to the underlying plotting routine throwing the error (plot_io_cost), which I haven't addressed here. I'm hesitant to fix there as it's possible we refactor those routines in the future, so just trying to keep things simple.

In the mean time, this should allow PyDarshan job summary to do something sensible for empty logs.

[shanedsnyder]

Example report output for empty log:

Open to any suggestions on the warning message.