metrics: report: quit cleanly on tidy_scaling failure

When there are no files to process, we tend to quit with a loud
and not helpful error. Improve that by spotting the obvious error
cases (such as no files to process for a specific test), and quit
with a nicer error/warning message that ends up in the rendered
report.

Start with the tidy_scaling test. The only clean way to quit a
fragment of Rmarkdown R looks to be to place it inside a function
so we can 'return'. Otherwise, all other forms of 'quit', quit the
whole Rmarkdown render pipeline, which is not what we want - we
want to carry on and try to process the rest of the fragments for
the rest of the tests.

Signed-off-by: Graham Whaley <graham.whaley@intel.com>

Author: Graham Whaley