Improve messaging for usage with no records

[nathanagood]

Version information

0.4.0
Darwin redacted.example.com. 18.7.0 Darwin Kernel Version 18.7.0: Thu Jan 23 06:52:12 PST 2020; root:xnu-4903.278.25~1/RELEASE_X86_64 x86_64
go version go1.13.8 darwin/amd64

Describe the bug

When I run the following command:

$ ./dce usage --start-date 1 --end-date `date +%s`     
err:  json: cannot unmarshal array into Go value of type operations.GetUsageOKBody
exit status 1

This looks like an unintended error. Expect an empty array

To Reproduce

Expected behavior

Additional context