Add warning message "No input data" for commandline tool when there are NO allure-result files at input directory

[amokmen]

Is your feature request related to a problem? Please describe.
As a Allure-newcomer I got 3 hours headache trying to understand why I see broken Allure-report in my browser (with "NaN" and "unknown" words). Problem was that I wrongfully understand what exactly serve do and point it to wrong directory without allure-results. But because of silent "normal" working of commandline tool - I thought, that problem is with my "badly" generated json-results files...

Describe the solution you'd like
Add at least WARNING MESSAGE to console, when commandline tool COULD NOT FIND any of appropriate input formats (for me it was json files). In other words - there should be message to poor user, which indicates, that tool now generating BROKEN/EMPTY report.