Fail with better error message if no terraform files exist in default location

[motatoes]

similar to #114 if user tries to connect a digger action and there is no digger.yml file AND no terraform files exist in the root directory then no errors are shown to the user instead the following error is shown:

terraform plan failed. dir: .
2023/04/07 04:39:35 Error executing plan: terraform plan failed. exit status 1
Error: Process completed with exit code 1.

We would like to improve this by showing a clearer error message

[fleroux514]

I can take this one

[motatoes]

Thank you! @fleroux514