roi should check for input files before reporting any problems with arguments

[andrewdavidsmith]

Describe the bug
The roi command will report problems with arguments, but likely should report on the existence of input files first. This is more immediate as some problems with arguments might depend on characteristics of the input files.

To Reproduce
Steps to reproduce the behavior:

1. Run dnmtools roi with a non-existent input file.

Expected behavior
The user should be informed that the file does not exist.

Screenshots
NA

Desktop (please complete the following information):

• OS: Ubuntu
• Version: 22

Additional context
Happens in v1.4.0