Cargo workspaces error verbosity

When attempting to run any `archway contracts` command outside of an unsupported project structure it returns `no such file or directory, open `. After asking around it seemed to be expecting a workspace toml.

I think these types of errors should be more verbose so the user has a better clue of what they might be doing wrong.

Run an `archway contracts` command outside a supported project structure and trace where the missing Cargo.toml error is produced. Improve the error so it explains the expected workspace structure and likely user mistake, then verify the command no longer reports only `no such file or directory`.

Written by the indexing model from the issue text.