The mediarequests API error message for files not found is quite confusing:
"The date(s) you used are valid, but we either do not have data for those date(s), or the project you asked for is not loaded yet. Please check https://wikimedia.org/api/rest_v1/?doc for more information."
This would make sense if it was the message that returned for a a valid media file that simply doesn't have page view data available yet (e.g. too new). But this is also the message returned if you look up a non-existent file name, or even give a completely impossible server location:
It should give an error such as "there is no such file," so you actually know if the input was invalid, and distinguishes from a data issue in the API.